[Android WebAPK] Fix NullPointerException when showing notification

This CL fixes NullPointerException when showing notifications in WebAPK.

BUG=779960

Change-Id: Icc4721995b57503e74071d7a1ae92f875d5de649
Reviewed-on: https://chromium-review.googlesource.com/748802Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Xi Han <hanxi@chromium.org>
Commit-Queue: Peter Kotwicz <pkotwicz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513186}

chrome/android/java/src/org/chromium/chrome/browser/webapps/WebApkServiceClient.java

@@ -31,6 +31,8 @@ public class WebApkServiceClient {
 
         @Override
         public void onConnected(IBinder api) {
+            if (api == null) return;
+
             try {
                 useApi(IWebApkApi.Stub.asInterface(api));
             } catch (RemoteException e) {