We mustn't finalize sweeping when V8 started collecting garbage.

third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp

Index: third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp
diff --git a/third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp b/third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp
index c3b4cc8eb022f32066119c36b08e2de4fc941a77..b142bff25b05b4ad618ad7f4d384ecf0aaec3c00 100644
--- a/third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp
+++ b/third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp
@@ -274,9 +274,6 @@ void gcPrologueForMajorGC(v8::Isolate* isolate, bool constructRetainedObjectInfo
 
 void V8GCController::gcPrologue(v8::GCType type, v8::GCCallbackFlags flags)
 {
-    if (ThreadState::current())
-        ThreadState::current()->willStartV8GC();
-

[haraken, 2015/11/20 15:11:35]

Would you move this to after ScriptForbiddenScope::enter() and comment out the
code with:

// TODO(haraken): It is not safe to run finalizers in a prologue callback
because V8AbstractEventListener's destructor can call into V8. We should post a
task to schedule willStartV8GC() and avoid running it inside the prologue
callback.

     if (isMainThread()) {
         ScriptForbiddenScope::enter();
     }