GuestView: Lazily push attributes to the browser process on attach

extensions/renderer/resources/guest_view/guest_view_attributes.js

Index: extensions/renderer/resources/guest_view/guest_view_attributes.js
diff --git a/extensions/renderer/resources/guest_view/guest_view_attributes.js b/extensions/renderer/resources/guest_view/guest_view_attributes.js
index a4c42b69b32a67c1437ff94f78d6e93843a70451..7bc33b81e24875e24ca0ad7ac2623fd8384381ab 100644
--- a/extensions/renderer/resources/guest_view/guest_view_attributes.js
+++ b/extensions/renderer/resources/guest_view/guest_view_attributes.js
@@ -9,9 +9,10 @@
 
 // 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();
 }
@@ -21,6 +22,15 @@ 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 || '');
@@ -51,6 +61,7 @@ Attribute.prototype.maybeHandleMutation = function(oldValue, newValue) {
   if (this.ignoreMutation)
     return;
 
+  this.dirty = true;
   this.handleMutation(oldValue, newValue);
 };