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

third_party/WebKit/Source/platform/ScriptForbiddenScope.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ScriptForbiddenScope_h
 #define ScriptForbiddenScope_h
 
 #include "platform/PlatformExport.h"
 #include "wtf/Allocator.h"
 #include "wtf/AutoReset.h"
@@ skipping 26 matching lines @@
 
    private:
     Optional<AutoReset<unsigned>> m_change;
   };
 
   static void enter() {
     DCHECK(isMainThread());
     ++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:
   static unsigned s_scriptForbiddenCount;
 };
 
 // Scoped disabling of script execution on the main thread,
 // if called on the main thread.
 //
 // No effect when used by from other threads -- simplifies
@@ skipping 13 matching lines @@
   ~ScriptForbiddenIfMainThreadScope() {
     if (m_IsMainThread)
       ScriptForbiddenScope::exit();
   }
   bool m_IsMainThread;
 };
 
 }  // namespace blink
 
 #endif  // ScriptForbiddenScope_h