Added plumbing for the availablechange event from Blink to WebPresentationClient.

public/web/WebFrameClient.h

Index: public/web/WebFrameClient.h
diff --git a/public/web/WebFrameClient.h b/public/web/WebFrameClient.h
index 62641d2a76f6e397b5b148989bc3f621fb8c0ce5..f27601e53064de169739d1c5e51e8c9b620b14ed 100644
--- a/public/web/WebFrameClient.h
+++ b/public/web/WebFrameClient.h
@@ -75,6 +75,7 @@ class WebServiceWorkerProvider;
 class WebSocketHandle;
 class WebPlugin;
 class WebPluginPlaceholder;
+class WebPresentationClient;
 class WebPushClient;
 class WebRTCPeerConnectionHandler;
 class WebScreenOrientationClient;
@@ -333,6 +334,12 @@ public:
     virtual WebPushClient* pushClient() { return 0; }
 
 
+    // Presentation API ----------------------------------------------------
+
+    // Used to access the embedder for the Presentation API.
+    virtual WebPresentationClient* presentationClient() { return 0; }
+
+
     // Editing -------------------------------------------------------------
 
     // These methods allow the client to intercept and overrule editing