Avoid calling isMainThread() in the common cases when checking ScriptForbiddenScope::isScriptForbid…

third_party/WebKit/Source/platform/ScriptForbiddenScope.h

Index: third_party/WebKit/Source/platform/ScriptForbiddenScope.h
diff --git a/third_party/WebKit/Source/platform/ScriptForbiddenScope.h b/third_party/WebKit/Source/platform/ScriptForbiddenScope.h
index b8ddadf0c4abd882d3f5c78575f1a99a717e3240..4aa19b5ff5e1ef72fd95d04ea849913d9a2bc385 100644
--- a/third_party/WebKit/Source/platform/ScriptForbiddenScope.h
+++ b/third_party/WebKit/Source/platform/ScriptForbiddenScope.h
@@ -44,11 +44,15 @@ class PLATFORM_EXPORT ScriptForbiddenScope final {
     ++s_scriptForbiddenCount;
   }
   static void exit() {
+    DCHECK(isMainThread());
     DCHECK(s_scriptForbiddenCount);
     --s_scriptForbiddenCount;
   }
   static bool isScriptForbidden() {
-    return isMainThread() && s_scriptForbiddenCount;
+    // Check s_scriptForbiddenCount first to avoid any calls to isMainThread()
+    // since under normal situations where we check this (ex. inside
+    // V8ScriptRunner) the value should always be zero.
+    return s_scriptForbiddenCount && isMainThread();
   }
 
  private: