Improve ScriptForbiddenScope handling in cross-threaded code.

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

Index: third_party/WebKit/Source/platform/heap/HeapPage.cpp
diff --git a/third_party/WebKit/Source/platform/heap/HeapPage.cpp b/third_party/WebKit/Source/platform/heap/HeapPage.cpp
index 136413569200bb7bcb158e12db2e3063881a04cb..40fe3cfdaedfa69b43b52e586abb9ded97bfac19 100644
--- a/third_party/WebKit/Source/platform/heap/HeapPage.cpp
+++ b/third_party/WebKit/Source/platform/heap/HeapPage.cpp
@@ -298,17 +298,11 @@ Address BaseHeap::lazySweep(size_t allocationSize, size_t gcInfoIndex)
         return nullptr;
 
     TRACE_EVENT0("blink_gc", "BaseHeap::lazySweepPages");
-    ThreadState::SweepForbiddenScope scope(threadState());
-    double startTime = WTF::currentTimeMS();
-
-    if (threadState()->isMainThread())
-        ScriptForbiddenScope::enter();
+    ThreadState::SweepForbiddenScope sweepForbidden(threadState());
+    ScriptForbiddenIfMainThreadScope scriptForbidden;
 
+    double startTime = WTF::currentTimeMS();
     Address result = lazySweepPages(allocationSize, gcInfoIndex);
-
-    if (threadState()->isMainThread())
-        ScriptForbiddenScope::exit();
-
     threadState()->accumulateSweepingTime(WTF::currentTimeMS() - startTime);
     Heap::reportMemoryUsageForTracing();