Hold global GC heap lock while making audio thread access.

third_party/WebKit/Source/platform/heap/ThreadState.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 1435 matching lines @@
 
   Optional<RuntimeCallTimerScope> timer_scope;
   if (v8::Isolate* isolate = GetIsolate()) {
     timer_scope.emplace(RuntimeCallStats::From(isolate),
                         RuntimeCallStats::CounterId::kCollectGarbage);
   }
 
   GCForbiddenScope gc_forbidden_scope(this);
 
   {
-    // Access to the CrossThreadPersistentRegion has to be prevented while in
-    // the marking phase because otherwise other threads may allocate or free
-    // PersistentNodes and we can't handle that.
+    // Access to the CrossThreadPersistentRegion has to be prevented
+    // while in the marking phase because otherwise other threads may
+    // allocate or free PersistentNodes and we can't handle
+    // that. Grabbing this lock also prevents non-attached threads
+    // from accessing any GCed heap while a GC runs.
     CrossThreadPersistentRegion::LockScope persistent_lock(
         ProcessHeap::GetCrossThreadPersistentRegion());
     {
       SafePointScope safe_point_scope(stack_state, this);
 
       std::unique_ptr<Visitor> visitor;
       if (gc_type == BlinkGC::kTakeSnapshot) {
         visitor = Visitor::Create(this, Visitor::kSnapshotMarking);
       } else {
         DCHECK(gc_type == BlinkGC::kGCWithSweep ||
@@ skipping 100 matching lines @@
     CollectGarbage(BlinkGC::kNoHeapPointersOnStack, BlinkGC::kGCWithSweep,
                    BlinkGC::kForcedGC);
     size_t live_objects = Heap().HeapStats().MarkedObjectSize();
     if (live_objects == previous_live_objects)
       break;
     previous_live_objects = live_objects;
   }
 }
 
 }  // namespace blink