Add stack direction heap unit test.

Source/platform/heap/StackFrameDepth.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef StackFrameDepth_h
 #define StackFrameDepth_h
 
 #include "platform/PlatformExport.h"
 #include "wtf/Assertions.h"
 #include <stdint.h>
 
 namespace blink {
 
 // StackFrameDepth keeps track of current call stack frame depth.
 // Use isSafeToRecurse() to query if there is a room in current
 // call stack for more recursive call.
 class PLATFORM_EXPORT StackFrameDepth final {
 public:
     inline bool isSafeToRecurse()
     {
         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;
     }
 
     static uintptr_t currentStackFrame(const char* dummy = nullptr)
     {
 #if COMPILER(GCC)
         return reinterpret_cast<uintptr_t>(__builtin_frame_address(0));
 #elif COMPILER(MSVC)
         return reinterpret_cast<uintptr_t>(&dummy) - sizeof(void*);
 #else
 #error "Stack frame pointer estimation not supported on this platform."
         return 0;
 #endif
     }
 
     void configureLimit();
 
 private:
     // The maximum depth of eager, unrolled trace() calls that is
     // considered safe and allowed.
     static const int kSafeStackFrameSize = 32 * 1024;
 
     uintptr_t m_stackFrameLimit;
 };
 
 } // namespace blink
 
 #endif // StackFrameDepth_h