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

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

Index: chrome/browser/extensions/api/declarative_content/declarative_content_apitest.cc
diff --git a/chrome/browser/extensions/api/declarative_content/declarative_content_apitest.cc b/chrome/browser/extensions/api/declarative_content/declarative_content_apitest.cc
index 798c4f488fa285bf389814e59974b969f0aa9722..74687f6169fe4d059603b8e680ffcb4e3d41184a 100644
--- a/chrome/browser/extensions/api/declarative_content/declarative_content_apitest.cc
+++ b/chrome/browser/extensions/api/declarative_content/declarative_content_apitest.cc
@@ -96,8 +96,9 @@ IN_PROC_BROWSER_TEST_F(DeclarativeContentApiTest, Overview) {
   // 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.";