Introduce ContextObserver

third_party/WebKit/Source/core/dom/ContextLifecycleObserver.h

Index: third_party/WebKit/Source/core/dom/ContextLifecycleObserver.h
diff --git a/third_party/WebKit/Source/core/dom/ContextLifecycleObserver.h b/third_party/WebKit/Source/core/dom/ContextLifecycleObserver.h
index a2fddd3a8a0a455b98b1fea132826f8ded0d4213..91eb2cdebf97e5276e260b3cda0d8bc58f363d0b 100644
--- a/third_party/WebKit/Source/core/dom/ContextLifecycleObserver.h
+++ b/third_party/WebKit/Source/core/dom/ContextLifecycleObserver.h
@@ -35,6 +35,31 @@ namespace blink {
 
 class LocalFrame;
 
+// ContextClient is a lightweight version of ContextLifecycleObserver.
+// If you just want to use getExecutionContext() and frame(), use

[dcheng, 2016/12/20 07:20:43]

Nit: I've heard some people prefer to avoid pronouns in comments.

One alternative wording:
ContextClient and ContextLifecycleObserver are helpers to associate a
class with an ExecutionContext. ContextLifecycleObserver provides an
additional contextDestroyed() hook to execute cleanup code when a
context is destroyed. Prefer the simpler ContextClient when possible.

+// ContextClient. If you need to listen contextDestroyed() notifications,
+// use ContextLifecycleObserver.
+class CORE_EXPORT ContextClient : public GarbageCollectedMixin {
+ public:
+  // Returns null after the observing context is detached.
+  ExecutionContext* getExecutionContext() const;
+
+  // Returns null after the observing context is detached or if the context
+  // doesn't have a frame (i.e., if the context is not a Document).
+  LocalFrame* frame() const;
+
+  DECLARE_VIRTUAL_TRACE();
+
+ protected:
+  explicit ContextClient(ExecutionContext* executionContext)
+      : m_executionContext(executionContext) {}
+
+  explicit ContextClient(LocalFrame*);
+
+ private:
+  WeakMember<ExecutionContext> m_executionContext;
+};
+
 class CORE_EXPORT ContextLifecycleObserver
     : public LifecycleObserver<ExecutionContext, ContextLifecycleObserver> {
  public: