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

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 745af03f16c7b18d6bb68db7cb6d5cef2ac6dbb1..264817c5df2da44f95c0922ddbcff8d58f0c98a8 100644
--- a/extensions/renderer/resources/guest_view/web_view_attributes.js
+++ b/extensions/renderer/resources/guest_view/web_view_attributes.js
@@ -197,6 +197,8 @@ SrcAttribute.prototype.handleMutation = function(oldValue, newValue) {
     // 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.
+    // takeRecords() needed to clear queued up src mutations.

[Fady Samuel, 2014/11/10 19:25:39]

This doesn't explain why this was causing an issue. Could you add further
clarification?

[paulmeyer, 2014/11/10 19:35:48]

Done.

+    this.observer.takeRecords();
     this.setValueIgnoreMutation(oldValue);
     return;
   }