Add stack direction heap unit test.

Source/platform/heap/StackFrameDepth.h

Index: Source/platform/heap/StackFrameDepth.h
diff --git a/Source/platform/heap/StackFrameDepth.h b/Source/platform/heap/StackFrameDepth.h
index ed43873ed34980243d0dd379160111fb885c86f1..8ad2b9d984d7a542566262f7e828e5d7e0ceb431 100644
--- a/Source/platform/heap/StackFrameDepth.h
+++ b/Source/platform/heap/StackFrameDepth.h
@@ -20,8 +20,11 @@ public:
     {
         ASSERT(m_stackFrameLimit);
 
-        // Below comparison assumes callstack to glow downward,
-        // which is true for all ABI Blink currently supports.
+        // Asssume that the stack grows towards lower addresses, which
+        // all the ABIs currently supported do.
+        //
+        // A unit test checks that the assumption holds for a target
+        // (HeapTest.StackGrowthDirection.)
         return currentStackFrame() > m_stackFrameLimit;
     }