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

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 1ec04816871f8000ba7032654b71ae7bcb9f0802..2c7a9a8a7fc40fe0d635ef4f986f66d137cf48e9 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -3844,6 +3844,8 @@ typedef void (*MemoryAllocationCallback)(ObjectSpace space,
                                          AllocationAction action,
                                          int size);
 
+typedef uint64_t (*TotalPhysicalMemoryCallback)();
+
 // --- Leave Script Callback ---
 typedef void (*CallCompletedCallback)();
 
@@ -4467,10 +4469,16 @@ class V8_EXPORT V8 {
   static void SetCreateHistogramFunction(CreateHistogramCallback);
   static void SetAddHistogramSampleFunction(AddHistogramSampleCallback);
 
-  /** Callback function for reporting failed access checks.*/
+  /** Callback function for reporting failed access checks. */
   static void SetFailedAccessCheckCallbackFunction(FailedAccessCheckCallback);
 
   /**
+   * Enables the host application to provide a function which returns the total
+   * amount of physical memory available on the platform.
+   */
+  static void SetTotalPhysicalMemoryFunction(TotalPhysicalMemoryCallback);
+
+  /**
    * Enables the host application to receive a notification before a
    * garbage collection.  Allocations are not allowed in the
    * callback function, you therefore cannot manipulate objects (set