Try forbidding GC to investigate MutationObserver crash

The iterator seems to break while iterating MutationObserverInterestGroup::observers_.
To figure out if this is conservative GC related, this CL adds a GC forbidden scope.

Bug: 1003733
Change-Id: I6511e1fef12771c5ed2d6891e547807151e89aa1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1883460Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710298}

third_party/blink/renderer/core/dom/mutation_observer_interest_group.cc

@@ -71,6 +71,11 @@ bool MutationObserverInterestGroup::IsOldValueRequested() {
 void MutationObserverInterestGroup::EnqueueMutationRecord(
     MutationRecord* mutation) {
   MutationRecord* mutation_with_null_old_value = nullptr;
+
+  // For investigation of crbug.com/1003733.
+  // If the crashes stop it means there is a GC related issue.
+  ThreadState::GCForbiddenScope gc_forbidden(ThreadState::Current());
+
   for (auto& iter : observers_) {
     MutationObserver* observer = iter.key.Get();
     if (HasOldValue(iter.value)) {