Provide a mechanism for the embedder to set a TotalPhysicalMemory callback function.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 6693b578f5aaaffe5396d9550a3627a4d1853234..24d482950f62f080cb61acac45ad6903f9b29703 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -287,6 +287,9 @@ class ThreadLocalTop BASE_EMBEDDED {
   // Call back function to report unsafe JS accesses.
   v8::FailedAccessCheckCallback failed_access_check_callback_;
 
+  // Call back function to get total physical memory of the platform.
+  v8::TotalPhysicalMemoryCallback total_physical_memory_callback_;
+
   // Head of the list of live LookupResults.
   LookupResult* top_lookup_result_;
 
@@ -773,6 +776,8 @@ class Isolate {
   void SetFailedAccessCheckCallback(v8::FailedAccessCheckCallback callback);
   void ReportFailedAccessCheck(JSObject* receiver, v8::AccessType type);
 
+  void SetTotalPhysicalMemoryCallback(v8::TotalPhysicalMemoryCallback callback);
+
   // Exception throwing support. The caller should use the result
   // of Throw() as its return value.
   Failure* Throw(Object* exception, MessageLocation* location = NULL);