Added plumbing for the availablechange event from Blink to WebPresentationClient.

Source/modules/presentation/Presentation.h

Index: Source/modules/presentation/Presentation.h
diff --git a/Source/modules/presentation/Presentation.h b/Source/modules/presentation/Presentation.h
index f075f735defa029934d85005b0dfbde8dcd4e0ec..ab390a9678623cc0b12ff8b7bd0b9fbfdbf924a1 100644
--- a/Source/modules/presentation/Presentation.h
+++ b/Source/modules/presentation/Presentation.h
@@ -6,22 +6,26 @@
 #define Presentation_h
 
 #include "bindings/core/v8/ScriptPromise.h"
-#include "core/dom/ContextLifecycleObserver.h"
 #include "core/events/EventTarget.h"
+#include "core/frame/FrameDestructionObserver.h"
 #include "modules/presentation/PresentationSession.h"
 
 namespace blink {
 
+class LocalFrame;
+class PresentationController;
 class ScriptState;
 
+// Implements the main entry point of the Presentation API corresponding to the Presentation.idl
+// See https://w3c.github.io/presentation-api/#navigatorpresentation for details.
 class Presentation final
     : public RefCountedGarbageCollectedEventTargetWithInlineData<Presentation>
-    , public ContextLifecycleObserver {
+    , public FrameDestructionObserver {
     DEFINE_EVENT_TARGET_REFCOUNTING_WILL_BE_REMOVED(RefCountedGarbageCollected<Presentation>);
     WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(Presentation);
     DEFINE_WRAPPERTYPEINFO();
 public:
-    static Presentation* create(ExecutionContext*);
+    static Presentation* create(LocalFrame*);
     virtual ~Presentation();
 
     // EventTarget implementation.
@@ -37,8 +41,22 @@ public:
 
     DEFINE_ATTRIBUTE_EVENT_LISTENER(availablechange);
 
+    // The embedder needs to keep track if anything is listening to the event so it could stop the
+    // might be expensive screen discovery process.
+    virtual bool addEventListener(const AtomicString& eventType, PassRefPtr<EventListener>, bool useCapture) override;
+    virtual bool removeEventListener(const AtomicString& eventType, PassRefPtr<EventListener>, bool useCapture) override;
+    virtual void removeAllEventListeners() override;
+
+    // Called when the |availablechange| event needs to be fired.
+    void didChangeAvailability(bool available);
+    // Queried by the controller if |availablechange| event has any listeners.
+    bool isAvailableChangeWatched() const;
 private:
-    explicit Presentation(ExecutionContext*);
+    explicit Presentation(LocalFrame*);
+
+    // Returns the |PresentationController| object associated with the frame |Presentation| corresponds to.
+    // Can return |nullptr| if the frame is detached from the document.
+    PresentationController* presentationController();
 
     Member<PresentationSession> m_session;
 };