Open a HandleScope before NearHeapLimitCallback

This should fix the recent frequent pause-on-oom failures.
The callback attempted to create a handle without a HandleScope.

Bug: v8:7631
Change-Id: Id15ba287896d62d3205585d14204f0c92a8823f8
Reviewed-on: https://chromium-review.googlesource.com/1030211Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52809}

src/heap/heap.cc

@@ -3470,6 +3470,7 @@ void Heap::RemoveNearHeapLimitCallback(v8::NearHeapLimitCallback callback,
 
 bool Heap::InvokeNearHeapLimitCallback() {
   if (near_heap_limit_callbacks_.size() > 0) {
+    HandleScope scope(isolate());
     v8::NearHeapLimitCallback callback =
         near_heap_limit_callbacks_.back().first;
     void* data = near_heap_limit_callbacks_.back().second;