Use Blink support to watch CSS selectors directly instead of using a MutationObserver.

chrome/browser/extensions/api/declarative_content/declarative_content_apitest.cc

 // Copyright (c) 2012 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.
 
 #include "chrome/browser/extensions/extension_action.h"
 #include "chrome/browser/extensions/extension_action_manager.h"
 #include "chrome/browser/extensions/extension_apitest.h"
 #include "chrome/browser/extensions/extension_tab_util.h"
 #include "chrome/browser/extensions/extension_test_message_listener.h"
 #include "chrome/browser/extensions/test_extension_dir.h"
@@ skipping 78 matching lines @@
   // than waiting for the extension to run.
   EXPECT_TRUE(page_action->GetIsVisible(tab_id));
 
   // Make sure leaving a matching page unshows the page action.
   NavigateInRenderer(tab, GURL("http://not_checked/"));
   EXPECT_FALSE(page_action->GetIsVisible(tab_id));
 
   // Insert a password field to make sure that's noticed.
   ASSERT_TRUE(content::ExecuteScript(
       tab, "document.body.innerHTML = '<input type=\"password\">';"));
-  // Give the mutation observer a chance to run and send back the
-  // matching-selector update.
+  // Give the style match a chance to run and send back the matching-selector
+  // update.  This takes two times through the event loop.
+  ASSERT_TRUE(content::ExecuteScript(tab, std::string()));
   ASSERT_TRUE(content::ExecuteScript(tab, std::string()));

[Matt Perry, 2013/10/02 22:20:22]

This double call looks fishy. Are you just calling it again to force a
RunMessageLoop()?

[Jeffrey Yasskin, 2013/10/03 00:31:57]

It's to force a RunMessageLoop in the Blink process. That's icky, but the
technique using Notifications was flaky.

   EXPECT_TRUE(page_action->GetIsVisible(tab_id))
       << "Adding a matching element should show the page action.";
 
   // Remove it again to make sure that reverts the action.
   ASSERT_TRUE(content::ExecuteScript(
       tab, "document.body.innerHTML = 'Hello world';"));
   // Give the mutation observer a chance to run and send back the
   // matching-selector update.
   ASSERT_TRUE(content::ExecuteScript(tab, std::string()));
   EXPECT_FALSE(page_action->GetIsVisible(tab_id))
@@ skipping 114 matching lines @@
                   "  new PageStateMatcher({css: ['div input']});\n"
                   "  Return('Failed to throw');\n"
                   "} catch (e) {\n"
                   "  Return(e.message);\n"
                   "}\n"),
               testing::ContainsRegex("compound selector.*: div input$"));
 }
 
 }  // namespace
 }  // namespace extensions