<webview> + <extensionview>: Create a maybeHandleMutation call into attributes

extensions/renderer/resources/guest_view/web_view_attributes.js

Index: extensions/renderer/resources/guest_view/web_view_attributes.js
diff --git a/extensions/renderer/resources/guest_view/web_view_attributes.js b/extensions/renderer/resources/guest_view/web_view_attributes.js
index f8077b9039dd5f0589a54eff697a6d734e47bf8d..41bbe952a648e6f4348221df5d1dfb2e682de45f 100644
--- a/extensions/renderer/resources/guest_view/web_view_attributes.js
+++ b/extensions/renderer/resources/guest_view/web_view_attributes.js
@@ -53,6 +53,15 @@ WebViewAttribute.prototype.defineProperty = function() {
 };
 
 // Called when the attribute's value changes.
+WebViewAttribute.prototype.maybeHandleMutation = function(oldValue, newValue) {
+  if (this.ignoreMutation) {
+    return;
+  }
+
+  this.handleMutation(oldValue, newValue);
+};
+
+// Called when a change that isn't ignored occurs to the attribute's value.
 WebViewAttribute.prototype.handleMutation = function(oldValue, newValue) {};
 
 // An attribute that is treated as a Boolean.