Avoid unsafe heap access from audio thread.

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

Index: third_party/WebKit/Source/platform/heap/ThreadState.cpp
diff --git a/third_party/WebKit/Source/platform/heap/ThreadState.cpp b/third_party/WebKit/Source/platform/heap/ThreadState.cpp
index 4ea6db5c7d45051815566d7a6cc21cd07fb41cf1..25401b83b88330b8f04e0b4511ae2eff404793ea 100644
--- a/third_party/WebKit/Source/platform/heap/ThreadState.cpp
+++ b/third_party/WebKit/Source/platform/heap/ThreadState.cpp
@@ -1440,6 +1440,12 @@ void ThreadState::CollectGarbage(BlinkGC::StackState stack_state,
   GCForbiddenScope gc_forbidden_scope(this);
 
   {
+    // No access to GCing arenas is allowed by other threads while
+    // a GC runs; grab a lock to indicate that GC is underway. Should
+    // any other thread need to access object on the heap, it will have
+    // to be while in a GCLockScope also.
+    GCLockScope gc_lock(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.