Add WaitForInterstitialReady to browser_test_utils.

content/public/test/browser_test_utils.h

Index: content/public/test/browser_test_utils.h
diff --git a/content/public/test/browser_test_utils.h b/content/public/test/browser_test_utils.h
index 761b5075eacaef16e61c7eece5befa74cae58e6d..c065fe8a491d1fd889bd1ef1c30734dae231ee39 100644
--- a/content/public/test/browser_test_utils.h
+++ b/content/public/test/browser_test_utils.h
@@ -52,6 +52,7 @@ class EmbeddedTestServer;
 namespace content {
 
 class BrowserContext;
+class InterstitialPage;
 class MessageLoopRunner;
 class RenderViewHost;
 class WebContents;
@@ -243,6 +244,9 @@ void WaitForInterstitialDetach(content::WebContents* web_contents);
 void RunTaskAndWaitForInterstitialDetach(content::WebContents* web_contents,
                                          const base::Closure& task);
 
+// Waits until all resources have loaded in an interstitial page.
+bool WaitForInterstitialReady(content::InterstitialPage* interstitial);
+
 // Watches title changes on a WebContents, blocking until an expected title is
 // set.
 class TitleWatcher : public WebContentsObserver {