Bug 1171282 - Avoid some unnecessary |operator new| null-checks in layout/. r=dholbert.

AllocateByObjectID() is infallible. Therefore the |operator new| of nsFrameList,
nsLineBox and nsRuleNode are too, as is nsRuleNode::CreateRootNode().

The patch also removes a couple of comments duplicated in both .h and .cpp
files.

layout/generic/nsFrameList.h

@@ -73,7 +73,7 @@ public:
   }
 
   /**
-   * Allocate a nsFrameList from the shell arena.
+   * Infallibly allocate a nsFrameList from the shell arena.
    */
   void* operator new(size_t sz, nsIPresShell* aPresShell) CPP_THROW_NEW;
 

layout/generic/nsLineBox.cpp

@@ -145,8 +145,6 @@ nsLineBox::NoteFramesMovedFrom(nsLineBox* aFromLine)
   }
 }
 
-// Overloaded new operator. Uses an arena (which comes from the presShell)
-// to perform the allocation.
 void*
 nsLineBox::operator new(size_t sz, nsIPresShell* aPresShell) CPP_THROW_NEW
 {

layout/generic/nsLineBox.h

@@ -206,8 +206,8 @@ private:
   nsLineBox(nsIFrame* aFrame, int32_t aCount, bool aIsBlock);
   ~nsLineBox();
   
-  // Overloaded new operator. Uses an arena (which comes from the presShell)
-  // to perform the allocation.
+  // Infallible overloaded new operator. Uses an arena (which comes from the
+  // presShell) to perform the allocation.
   void* operator new(size_t sz, nsIPresShell* aPresShell) CPP_THROW_NEW;
   void operator delete(void* aPtr, size_t sz) = delete;
 

layout/style/nsRuleNode.cpp

@@ -1394,7 +1394,6 @@ SetFactor(const nsCSSValue& aValue, float& aField, bool& aCanStoreInRuleTree,
   NS_NOTREACHED("SetFactor: inappropriate unit");
 }
 
-// Overloaded new operator that allocates from a presShell arena.
 void*
 nsRuleNode::operator new(size_t sz, nsPresContext* aPresContext) CPP_THROW_NEW
 {
@@ -1554,20 +1553,11 @@ nsRuleNode::Transition(nsIStyleRule* aRule, uint8_t aLevel,
     else {
       next = entry->mRuleNode = new (mPresContext)
         nsRuleNode(mPresContext, this, aRule, aLevel, aIsImportantRule);
-      if (!next) {
-        PL_DHashTableRawRemove(ChildrenHash(), entry);
-        NS_WARNING("out of memory");
-        return this;
-      }
     }
   } else if (!next) {
     // Create the new entry in our list.
     next = new (mPresContext)
       nsRuleNode(mPresContext, this, aRule, aLevel, aIsImportantRule);
-    if (!next) {
-      NS_WARNING("out of memory");
-      return this;
-    }
     next->mNextSibling = ChildrenList();
     SetChildrenList(next);
   }

layout/style/nsRuleNode.h

@@ -394,7 +394,7 @@ private:
   uint32_t mRefCnt;
 
 public:
-  // Overloaded new operator that allocates from a presShell arena.
+  // Infallible overloaded new operator that allocates from a presShell arena.
   void* operator new(size_t sz, nsPresContext* aContext) CPP_THROW_NEW;
   void Destroy() { DestroyInternal(nullptr); }
 
@@ -651,6 +651,7 @@ private:
   ~nsRuleNode();
 
 public:
+  // This is infallible; it will never return nullptr.
   static nsRuleNode* CreateRootNode(nsPresContext* aPresContext);
 
   static void EnsureBlockDisplay(uint8_t& display,

layout/style/nsStyleSet.cpp

@@ -218,10 +218,7 @@ nsStyleSet::BeginReconstruct()
   NS_ASSERTION(mRuleTree, "Reconstructing before first construction?");
 
   // Create a new rule tree root
-  nsRuleNode* newTree =
-    nsRuleNode::CreateRootNode(mRuleTree->PresContext());
-  if (!newTree)
-    return NS_ERROR_OUT_OF_MEMORY;
+  nsRuleNode* newTree = nsRuleNode::CreateRootNode(mRuleTree->PresContext());
 
   // Save the old rule tree so we can destroy it later
   if (!mOldRuleTrees.AppendElement(mRuleTree)) {