Bug 414087 (part 2) - Remove insignificant wasted space in StackBlock. r=bz.

layout/base/StackArena.cpp

@@ -10,11 +10,10 @@ namespace mozilla {
 
 // A block of memory that the stack will chop up and hand out.
 struct StackBlock {
-  // A bit under 4096, for malloc overhead.
-  static const size_t MAX_USABLE_SIZE = 4044;
+  // Subtract sizeof(StackBlock*) to give space for the |mNext| field.
+  static const size_t MAX_USABLE_SIZE = 4096 - sizeof(StackBlock*);
 
-  // A block of memory.  Note that this must be first so that it will
-  // be aligned.
+  // A block of memory.
   char mBlock[MAX_USABLE_SIZE];
 
   // Another block of memory that would only be created if our stack
@@ -25,6 +24,8 @@ struct StackBlock {
   ~StackBlock() { }
 };
 
+static_assert(sizeof(StackBlock) == 4096, "StackBlock must be 4096 bytes");
+
 // We hold an array of marks. A push pushes a mark on the stack.
 // A pop pops it off.
 struct StackMark {

layout/base/StackArena.h

@@ -82,7 +82,6 @@ public:
   }
 
   static void* Allocate(size_t aSize) {
-    MOZ_ASSERT(aSize <= 4044);
     return gStackArena->Allocate(aSize);
   }