[heap] Safety check for {CodePageMemoryModificationScope}.

R=hpayer@chromium.org
BUG=v8:6792,chromium:774108

Change-Id: I202993ddf5bb11d238ed173197657c6546dd4a37
Reviewed-on: https://chromium-review.googlesource.com/789865Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49675}

src/heap/heap.h

@@ -2658,6 +2658,10 @@ class CodePageMemoryModificationScope {
  private:
   MemoryChunk* chunk_;
   bool scope_active_;
+
+  // Disallow any GCs inside this scope, as a relocation of the underlying
+  // object would change the {MemoryChunk} that this scope targets.
+  DisallowHeapAllocation no_heap_allocation_;
 };
 
 // Visitor class to verify interior pointers in spaces that do not contain