Prevent dialog from invalid context

  Bug: 6346748

Change-Id: Ia65ecc1ecb882b480127aca0034db25a9e51ed3c

Author: Michael Kolb

core/java/android/webkit/WebCoreThreadWatchdog.java

@@ -16,6 +16,7 @@
 
 package android.webkit;
 
+import android.app.Activity;
 import android.app.AlertDialog;
 import android.content.Context;
 import android.content.DialogInterface;
@@ -146,6 +147,7 @@ public void handleMessage(Message msg) {
                         break;
 
                     case TIMED_OUT:
+                        if ((mContext == null) || !(mContext instanceof Activity)) return;
                         new AlertDialog.Builder(mContext)
                             .setMessage(com.android.internal.R.string.webpage_unresponsive)
                             .setPositiveButton(com.android.internal.R.string.force_close,