Move WebView.captureVisibleRegion to stable API.

extensions/renderer/resources/guest_view/web_view/web_view_experimental.js

-// Copyright 2015 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.
-
-// This module implements experimental API for <webview>.
-// See web_view.js and web_view_api_methods.js for details.
-//
-// <webview> Experimental API is only available on canary and channels of
-// Chrome.
-
-var WebViewImpl = require('webView').WebViewImpl;
-var WebViewInternal = require('webViewInternal').WebViewInternal;
-
-// An array of <webview>'s experimental API methods.  See |WEB_VIEW_API_METHODS|
-// in web_view_api_methods.js for more details.
-var WEB_VIEW_EXPERIMENTAL_API_METHODS = [
-  // Captures the visible region of the WebView contents into a bitmap.
-  'captureVisibleRegion'
-];
-
-// Registers the experimantal WebVIew API when available.
-WebViewImpl.maybeGetExperimentalApiMethods = function() {
-  return WEB_VIEW_EXPERIMENTAL_API_METHODS;
-};