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..bc978f820db32ae3d9598e5f832b271c4164b428 100644
--- a/Source/modules/presentation/Presentation.h
+++ b/Source/modules/presentation/Presentation.h
@@ -12,8 +12,11 @@
 
 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 {
@@ -37,9 +40,21 @@ 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);
 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;
 };