heap: Prevent allocations during atomic pause marking

Allocations during the atomic pause were never supported.
Enforcing it using scopes helps debugging issues were this
invariant is violated.

Bug: 986235, 993415
Change-Id: I5fd8ecd147403e2a7a89185b55057550d135bac2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1869225
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707760}

third_party/blink/renderer/platform/heap/thread_state.cc

@@ -657,6 +657,7 @@ void ThreadState::AtomicPauseMarkPrologue(BlinkGC::StackState stack_state,
       ThreadHeapStatsCollector::kAtomicPauseMarkPrologue, "epoch", gc_age_,
       "forced", IsForcedGC(reason));
   EnterAtomicPause();
+  EnterNoAllocationScope();
   EnterGCForbiddenScope();
   // Compaction needs to be canceled when incremental marking ends with a
   // conservative GC.
@@ -1268,8 +1269,9 @@ void ThreadState::AtomicPauseMarkEpilogue(BlinkGC::MarkingType marking_type) {
       ThreadHeapStatsCollector::kAtomicPauseMarkEpilogue, "epoch", gc_age_,
       "forced", IsForcedGC(current_gc_data_.reason));
   MarkPhaseEpilogue(marking_type);
-  LeaveAtomicPause();
   LeaveGCForbiddenScope();
+  LeaveNoAllocationScope();
+  LeaveAtomicPause();
   static_cast<MutexBase&>(ProcessHeap::CrossThreadPersistentMutex()).unlock();
 }