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 c94d0f25416bdb12f23aa5c66af5a0bb63de5267..688f237b5d78e4b589e21f3eebe52fc6387b430e 100644
--- a/public/web/WebFrameClient.h
+++ b/public/web/WebFrameClient.h
@@ -76,6 +76,7 @@ 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