diff --git a/xpcom/string/public/nsStringBuffer.h b/xpcom/string/public/nsStringBuffer.h
index 83db7804c76..229ee050325 100644
--- a/xpcom/string/public/nsStringBuffer.h
+++ b/xpcom/string/public/nsStringBuffer.h
@@ -169,6 +169,11 @@ class nsStringBuffer
                            bool aMoveOwnership = false);
       void ToString(PRUint32 len, nsACString &str,
                            bool aMoveOwnership = false);
+
+      /**
+       * This measures the size only if the StringBuffer is unshared.
+       */
+      size_t SizeOfIncludingThisIfUnshared(nsMallocSizeOfFun aMallocSizeOf) const;
   };
 
 #endif /* !defined(nsStringBuffer_h__ */
diff --git a/xpcom/string/src/nsSubstring.cpp b/xpcom/string/src/nsSubstring.cpp
index d3842e2174a..65250834cd3 100644
--- a/xpcom/string/src/nsSubstring.cpp
+++ b/xpcom/string/src/nsSubstring.cpp
@@ -307,6 +307,16 @@ nsStringBuffer::ToString(PRUint32 len, nsACString &str,
     accessor->set(data, len, flags);
   }
 
+size_t
+nsStringBuffer::SizeOfIncludingThisIfUnshared(nsMallocSizeOfFun aMallocSizeOf) const
+  {
+    if (!IsReadonly())
+      {
+        return aMallocSizeOf(this, sizeof(nsStringBuffer) + mStorageSize);
+      }
+    return 0;
+  }
+
 // ---------------------------------------------------------------------------