Fixed the problem at the source of the flaky WebViewTest.ShimArcAttribute test.

extensions/renderer/resources/guest_view/web_view_attributes.js

 // Copyright (c) 2014 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 the attributes of the <webview> tag.
 
 var GuestViewInternal =
     require('binding').Binding.create('guestViewInternal').generate();
 var WebViewImpl = require('webView').WebViewImpl;
 var WebViewConstants = require('webViewConstants').WebViewConstants;
@@ skipping 171 matching lines @@
 };
 
 // Attribute that handles the location and navigation of the webview.
 function SrcAttribute(webViewImpl) {
   WebViewAttribute.call(this, WebViewConstants.ATTRIBUTE_SRC, webViewImpl);
   this.setupMutationObserver();
 }
 
 SrcAttribute.prototype.__proto__ = WebViewAttribute.prototype;
 
+SrcAttribute.prototype.setValueIgnoreMutation = function(value) {
+  // takeRecords() is needed to clear queued up src mutations. Without it, it is
+  // possible for this change to get picked up asyncronously by src's mutation
+  // observer |observer|, and then get handled even though we do not want to
+  // handle this mutation.
+  this.observer.takeRecords();
+  this.ignoreMutation = true;
+  this.webViewImpl.webviewNode.setAttribute(this.name, value || '');
+  this.ignoreMutation = false;
+}
+
 SrcAttribute.prototype.handleMutation = function(oldValue, newValue) {
   // Once we have navigated, we don't allow clearing the src attribute.
   // Once <webview> enters a navigated state, it cannot return to a
   // placeholder state.
   if (!newValue && oldValue) {
     // src attribute changes normally initiate a navigation. We suppress
     // the next src attribute handler call to avoid reloading the page
     // on every guest-initiated navigation.
     this.setValueIgnoreMutation(oldValue);
     return;
@@ skipping 44 matching lines @@
 
   var autosizeAttributes = [WebViewConstants.ATTRIBUTE_MAXHEIGHT,
                             WebViewConstants.ATTRIBUTE_MAXWIDTH,
                             WebViewConstants.ATTRIBUTE_MINHEIGHT,
                             WebViewConstants.ATTRIBUTE_MINWIDTH];
   for (var i = 0; autosizeAttributes[i]; ++i) {
     this.attributes[autosizeAttributes[i]] =
         new AutosizeDimensionAttribute(autosizeAttributes[i], this);
   }
 };