Pass the kGCCallbackFlagForced flag when invoking Heap::CollectAllGarbage from...

Pass the kGCCallbackFlagForced flag when invoking Heap::CollectAllGarbage from AdjustAmountOfExternalAllocatedMemory.

This forces the second pass of the pending phantom callbacks to run immediately after the first.

BUG=chromium:511294
LOG=Y

Review URL: https://codereview.chromium.org/1252993004

Cr-Commit-Position: refs/heads/master@{#29933}

src/api.cc

@@ -6885,7 +6885,7 @@ Local<Integer> v8::Integer::NewFromUnsigned(Isolate* isolate, uint32_t value) {
 
 void Isolate::CollectAllGarbage(const char* gc_reason) {
   reinterpret_cast<i::Isolate*>(this)->heap()->CollectAllGarbage(
-      i::Heap::kNoGCFlags, gc_reason);
+      i::Heap::kNoGCFlags, gc_reason, v8::kGCCallbackFlagForced);
 }