Added plumbing for the availablechange event from Blink to WebPresentationClient.

Source/modules/presentation/Presentation.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 Presentation_h
 #define Presentation_h
 
 #include "bindings/core/v8/ScriptPromise.h"
 #include "core/dom/ContextLifecycleObserver.h"
 #include "core/events/EventTarget.h"
 #include "modules/presentation/PresentationSession.h"
 
 namespace blink {
 
+class PresentationController;
 class ScriptState;
 
+// Implements the main entry point of the Presentation API corresponding to the Presentation.idl
+// See http://w3c.github.io/presentation-api/#navigatorpresentation for details.
 class Presentation final
     : public RefCountedGarbageCollectedEventTargetWithInlineData<Presentation>
     , public ContextLifecycleObserver {
     DEFINE_EVENT_TARGET_REFCOUNTING_WILL_BE_REMOVED(RefCountedGarbageCollected<Presentation>);
     WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(Presentation);
     DEFINE_WRAPPERTYPEINFO();
 public:
     static Presentation* create(ExecutionContext*);
     virtual ~Presentation();
 
     // EventTarget implementation.
     virtual const AtomicString& interfaceName() const override;
     virtual ExecutionContext* executionContext() const override;
 
     virtual void trace(Visitor*) override;
 
     PresentationSession* session() const;
 
     ScriptPromise startSession(ScriptState*, const String& senderId, const String& presentationId);
     ScriptPromise joinSession(ScriptState*, const String& senderId, const String& presentationId);
 
     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);
 private:
     explicit Presentation(ExecutionContext*);
 
+    // Returns the |PresentationController| object associated with the frame |Presentation| corresponds to.
+    // Must never return null.
+    PresentationController* presentationController();

[mlamouri (slow - plz ping), 2015/01/22 23:18:04]

Are you sure that this must _never_ return nullptr? It seems that it can when
the frame is detached. Could you check whether it is null before using the
PresentationController when you are not already sure to have a frame? I think
this might fix your tests failures.

[whywhat, 2015/01/23 19:20:19]

Done.

+
     Member<PresentationSession> m_session;
 };
 
 } // namespace blink
 
 #endif // Presentation_h