diff --git a/ipc/chromium/src/base/histogram.cc b/ipc/chromium/src/base/histogram.cc
index a6d3579957c..0b2a1e95742 100644
--- a/ipc/chromium/src/base/histogram.cc
+++ b/ipc/chromium/src/base/histogram.cc
@@ -962,6 +962,31 @@ FlagHistogram::Accumulate(Sample value, Count count, size_t index)
   Histogram::Accumulate(1, -1, zero_index);
 }
 
+void
+FlagHistogram::AddSampleSet(const SampleSet& sample) {
+  DCHECK_EQ(bucket_count(), sample.size());
+  // We can't be sure the SampleSet provided came from another FlagHistogram,
+  // so we take the following steps:
+  //  - If our flag has already been set do nothing.
+  //  - Set our flag if the following hold:
+  //      - The sum of the counts in the provided SampleSet is 1.
+  //      - The bucket index for that single value is the same as the index where we
+  //        would place our set flag.
+  //  - Otherwise, take no action.
+
+  if (mSwitched) {
+    return;
+  }
+
+  if (sample.sum() != 1) {
+    return;
+  }
+
+  size_t one_index = BucketIndex(1);
+  if (sample.counts(one_index) == 1) {
+    Accumulate(1, 1, one_index);
+  }
+}
 //------------------------------------------------------------------------------
 // CustomHistogram:
 //------------------------------------------------------------------------------
diff --git a/ipc/chromium/src/base/histogram.h b/ipc/chromium/src/base/histogram.h
index 1995ddbbaf3..621df384f14 100644
--- a/ipc/chromium/src/base/histogram.h
+++ b/ipc/chromium/src/base/histogram.h
@@ -336,6 +336,7 @@ class Histogram {
     Count TotalCount() const;
     int64 sum() const { return sum_; }
     int64 redundant_count() const { return redundant_count_; }
+    size_t size() const { return counts_.size(); }
 
     // Arithmetic manipulation of corresponding elements of the set.
     void Add(const SampleSet& other);
@@ -392,7 +393,7 @@ class Histogram {
     Add(static_cast<int>(time.InMilliseconds()));
   }
 
-  void AddSampleSet(const SampleSet& sample);
+  virtual void AddSampleSet(const SampleSet& sample);
 
   // This method is an interface, used only by LinearHistogram.
   virtual void SetRangeDescriptions(const DescriptionPair descriptions[]);
@@ -661,6 +662,8 @@ public:
 
   virtual void Accumulate(Sample value, Count count, size_t index);
 
+  virtual void AddSampleSet(const SampleSet& sample);
+
 private:
   explicit FlagHistogram(const std::string &name);
   bool mSwitched;
diff --git a/toolkit/components/telemetry/tests/unit/test_nsITelemetry.js b/toolkit/components/telemetry/tests/unit/test_nsITelemetry.js
index 3dda5c5a6a9..80cbd262029 100644
--- a/toolkit/components/telemetry/tests/unit/test_nsITelemetry.js
+++ b/toolkit/components/telemetry/tests/unit/test_nsITelemetry.js
@@ -132,15 +132,11 @@ function compareHistograms(h1, h2) {
   do_check_eq(s1.histogram_type, s2.histogram_type);
   do_check_eq(s1.min, s2.min);
   do_check_eq(s1.max, s2.max);
+  do_check_eq(s1.sum, s2.sum);
 
-  // XXX Don't compare flag sums until bug 747379 is fixed
-  if (s1.histogram_type != Telemetry.HISTOGRAM_FLAG) {
-    do_check_eq(s1.sum, s2.sum);
-
-    do_check_eq(s1.counts.length, s2.counts.length);
-    for (let i = 0; i < s1.counts.length; i++)
-      do_check_eq(s1.counts[i], s2.counts[i]);
-  }
+  do_check_eq(s1.counts.length, s2.counts.length);
+  for (let i = 0; i < s1.counts.length; i++)
+    do_check_eq(s1.counts[i], s2.counts[i]);
 
   do_check_eq(s1.ranges.length, s2.ranges.length);
   for (let i = 0; i < s1.ranges.length; i++)
@@ -148,7 +144,7 @@ function compareHistograms(h1, h2) {
 }
 
 function test_histogramFrom() {
-  // One histogram of each type
+  // Test one histogram of each type.
   let names = ["CYCLE_COLLECTOR", "GC_REASON", "GC_RESET", "TELEMETRY_TEST_FLAG"];
 
   for each (let name in names) {
@@ -157,6 +153,12 @@ function test_histogramFrom() {
     let clone = Telemetry.histogramFrom("clone" + name, name);
     compareHistograms(original, clone);
   }
+
+  // Additionally, set the flag on TELEMETRY_TEST_FLAG, and check it gets set on the clone.
+  let testFlag = Telemetry.getHistogramById("TELEMETRY_TEST_FLAG");
+  testFlag.add(1);
+  let clone = Telemetry.histogramFrom("FlagClone", "TELEMETRY_TEST_FLAG");
+  compareHistograms(testFlag, clone);
 }
 
 function test_getSlowSQL() {