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..2dcc0042323f753c12e8d09e4b65fcd417d78ed7 100644
--- a/third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp
+++ b/third_party/WebKit/Source/bindings/core/v8/V8GCController.cpp
@@ -274,13 +274,13 @@ void gcPrologueForMajorGC(v8::Isolate* isolate, bool constructRetainedObjectInfo
 
 void V8GCController::gcPrologue(v8::GCType type, v8::GCCallbackFlags flags)
 {
-    if (ThreadState::current())
-        ThreadState::current()->willStartV8GC();
-
     if (isMainThread()) {
         ScriptForbiddenScope::enter();
     }
 
+    if (ThreadState::current())
+        ThreadState::current()->willStartV8GC();
+
     // TODO(haraken): It would be nice if the GC callbacks passed the Isolate
     // directly.
     v8::Isolate* isolate = v8::Isolate::GetCurrent();