Move most of the implementation of AdjustAmountOfExternalMemory to v8.h

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index fdc6bf96f3921fd8275f8ec3645c5f826beed646..e5966efd8fec8c087442df54573470e218af212e 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -764,8 +764,9 @@ void Runtime::FreeArrayBuffer(Isolate* isolate,
   size_t allocated_length = NumberToSize(
       isolate, phantom_array_buffer->byte_length());
 
-  isolate->heap()->AdjustAmountOfExternalAllocatedMemory(
-      -static_cast<int64_t>(allocated_length));
+  reinterpret_cast<v8::Isolate*>(isolate)
+      ->AdjustAmountOfExternalAllocatedMemory(
+          -static_cast<int64_t>(allocated_length));
   CHECK(V8::ArrayBufferAllocator() != NULL);
   V8::ArrayBufferAllocator()->Free(
       phantom_array_buffer->backing_store(),
@@ -819,7 +820,8 @@ bool Runtime::SetupArrayBufferAllocatingData(
 
   SetupArrayBuffer(isolate, array_buffer, false, data, allocated_length);
 
-  isolate->heap()->AdjustAmountOfExternalAllocatedMemory(allocated_length);
+  reinterpret_cast<v8::Isolate*>(isolate)
+      ->AdjustAmountOfExternalAllocatedMemory(allocated_length);
 
   return true;
 }