GuestView: Lazily push attributes to the browser process on attach

extensions/renderer/resources/guest_view/guest_view_attributes.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 the base attributes of the GuestView tags.
 
 // -----------------------------------------------------------------------------
 // Attribute object.
 
 // Default implementation of a GuestView attribute.
 function Attribute(name, view) {
+  this.dirty = false;
+  this.ignoreMutation = false;
   this.name = name;
   this.view = view;
-  this.ignoreMutation = false;
 
   this.defineProperty();
 }
 
 // Retrieves and returns the attribute's value.
 Attribute.prototype.getValue = function() {
   return this.view.element.getAttribute(this.name) || '';
 };
 
+// Retrieves and returns the attribute's value if it has been dirtied since
+// the last time this method was called. Returns null otherwise.
+Attribute.prototype.getValueIfDirty = function() {
+  if (!this.dirty)
+    return null;
+  this.dirty = false;
+  return this.getValue();
+};
+
 // Sets the attribute's value.
 Attribute.prototype.setValue = function(value) {
   this.view.element.setAttribute(this.name, value || '');
 };
 
 // Changes the attribute's value without triggering its mutation handler.
 Attribute.prototype.setValueIgnoreMutation = function(value) {
   this.ignoreMutation = true;
   this.setValue(value);
   this.ignoreMutation = false;
@@ skipping 10 matching lines @@
     }.bind(this),
     enumerable: true
   });
 };
 
 // Called when the attribute's value changes.
 Attribute.prototype.maybeHandleMutation = function(oldValue, newValue) {
   if (this.ignoreMutation)
     return;
 
+  this.dirty = true;
   this.handleMutation(oldValue, newValue);
 };
 
 // Called when a change that isn't ignored occurs to the attribute's value.
 Attribute.prototype.handleMutation = function(oldValue, newValue) {};
 
 // Called when the view's element is attached to the DOM tree.
 Attribute.prototype.attach = function() {};
 
 // Called when the view's element is detached from the DOM tree.
@@ skipping 58 matching lines @@
 
 var GuestViewAttributes = {
   Attribute: Attribute,
   BooleanAttribute: BooleanAttribute,
   IntegerAttribute: IntegerAttribute,
   ReadOnlyAttribute: ReadOnlyAttribute
 };
 
 // Exports.
 exports.GuestViewAttributes = GuestViewAttributes;