Make WindowAndroid#getWindowToken protected (was private).

This will be used in ChromeCast to override window token used by
content/public/android/java/src/org/chromium/content/browser/androidoverlay.

Bug: None.
Change-Id: Ibc5a97c24acfad1d8ab8aef69ec8b821abc71854
Reviewed-on: https://chromium-review.googlesource.com/c/1333727
Commit-Queue: Jan Lamecki <jachor@google.com>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609365}

ui/android/java/src/org/chromium/ui/base/WindowAndroid.java

@@ -773,7 +773,7 @@ public class WindowAndroid implements AndroidPermissionDelegate {
      * Return the current window token, or null.
      */
     @CalledByNative
-    private IBinder getWindowToken() {
+    protected IBinder getWindowToken() {
         Activity activity = activityFromContext(mContextRef.get());
         if (activity == null) return null;
         Window window = activity.getWindow();