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..5010b9303ee222162606f332e8c3da356ae825e5 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;
 
+// ContextObserver is a lightweight version of ContextLifecycleObserver.
+// If you just want to use getExecutionContext() and frame(), use
+// ContextObserver. If you need to listen contextDestroyed() notifications,
+// use ContextLifecycleObserver.
+class CORE_EXPORT ContextObserver : 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 ContextObserver(ExecutionContext* executionContext)
+      : m_executionContext(executionContext) {}
+
+  explicit ContextObserver(LocalFrame*);
+
+ private:
+  WeakMember<ExecutionContext> m_executionContext;

[sof, 2016/12/19 09:14:03]

For clarity, what motivates the weakness?

[haraken, 2016/12/19 09:58:15]

For example, this CL (https://codereview.chromium.org/2555873004/) caused memory
leaks.

Currently it's not guaranteed that contextDestroyed is called for all contexts
e.g., documents created by DOMImplementation::createDocument. So we cannot
assume that m_executionConntext is cleared before the context is detached.

+};
+
 class CORE_EXPORT ContextLifecycleObserver
     : public LifecycleObserver<ExecutionContext, ContextLifecycleObserver> {
  public: