Remove deprecated class WebContentsObserverAndroid and cleanup existing usages.

content/public/android/java/src/org/chromium/content/browser/WebContentsObserverAndroid.java

-// Copyright 2012 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-package org.chromium.content.browser;
-
-import org.chromium.content_public.browser.WebContents;
-
-/**
- * @deprecated This class has been renamed to {@link WebContentsObserver} and
- * will be removed when all usages have been removed. See http://crbug.com/3961
- */
-@Deprecated
-public abstract class WebContentsObserverAndroid extends WebContentsObserver {
-
-    public WebContentsObserverAndroid(WebContents webContents) {
-        super(webContents);
-    }
-}