Add WaitForInterstitialReady to browser_test_utils.

content/public/test/browser_test_utils.cc

Index: content/public/test/browser_test_utils.cc
diff --git a/content/public/test/browser_test_utils.cc b/content/public/test/browser_test_utils.cc
index 89dd3430e96dbca0330338b1fc035951d215e908..20d6f6c956fa702995dbcfff22395ebb29ce7bd7 100644
--- a/content/public/test/browser_test_utils.cc
+++ b/content/public/test/browser_test_utils.cc
@@ -723,6 +723,28 @@ void RunTaskAndWaitForInterstitialDetach(content::WebContents* web_contents,
   loop_runner->Run();
 }
 
+bool WaitForRenderFrameReady(RenderFrameHost* rfh) {
+  if (!rfh)
+    return false;
+  std::string result;
+  EXPECT_TRUE(
+      content::ExecuteScriptAndExtractString(
+          rfh,
+          "(function() {"
+          "  var done = false;"
+          "  function checkState() {"
+          "    if (!done && document.readyState == 'complete') {"
+          "      done = true;"
+          "      window.domAutomationController.send('pageLoadComplete');"
+          "    }"
+          "  }"
+          "  checkState();"
+          "  document.addEventListener('readystatechange', checkState);"
+          "})();",
+          &result));
+  return result == "pageLoadComplete";
+}
+
 TitleWatcher::TitleWatcher(WebContents* web_contents,
                            const base::string16& expected_title)
     : WebContentsObserver(web_contents),