Bind Window focus ability to the ExecutionContext instead of the process.

Source/core/dom/ExecutionContext.h

Index: Source/core/dom/ExecutionContext.h
diff --git a/Source/core/dom/ExecutionContext.h b/Source/core/dom/ExecutionContext.h
index daf2172090d2ffd8344f385334d5884f0d19988c..95423dac7bdb8d6d53b428087ed9f9d93e75008d 100644
--- a/Source/core/dom/ExecutionContext.h
+++ b/Source/core/dom/ExecutionContext.h
@@ -136,6 +136,10 @@ public:
     void enforceStrictMixedContentChecking() { m_strictMixedContentCheckingEnforced = true; }
     bool shouldEnforceStrictMixedContentChecking() const { return m_strictMixedContentCheckingEnforced; }
 
+    void allowWindowFocus();
+    void consumeWindowFocus();
+    bool isWindowFocusAllowed() const;
+
 protected:
     ExecutionContext();
     virtual ~ExecutionContext();
@@ -182,6 +186,12 @@ private:
     // ExecutionContext's members (notably m_timeouts) is called before they are destructed,
     // m_lifecycleNotifer should be placed *after* such members.
     OwnPtr<ContextLifecycleNotifier> m_lifecycleNotifier;
+
+    // Counter that keeps track of how many window focus calls are allowed for
+    // this ExecutionContext. Callers are expected to call |allowWindowFocus()|
+    // and |consumeWindowFocus()| in order to increment and decrement the
+    // counter.
+    int m_windowFocusTokens;
 };
 
 } // namespace blink