(Hopefully) Fix flaky ChromeNavigationBrowserTest.RedirectErrorPageReloadToAboutBlank.

This CL tries to deflake
ChromeNavigationBrowserTest.RedirectErrorPageReloadToAboutBlank. The
theory is that the extension web request listeners aren't getting set up
by the time the page is reloaded, hence the redirect is not happening.
Fix by waiting for an ack from the extension background page.

BUG=981295

Change-Id: If1171578dcbf248b910cbb6f774491ccb348f2ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2056942Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743241}

chrome/browser/chrome_navigation_browsertest.cc

@@ -53,6 +53,7 @@
 #include "content/public/test/navigation_handle_observer.h"
 #include "content/public/test/test_navigation_observer.h"
 #include "content/public/test/url_loader_interceptor.h"
+#include "extensions/test/extension_test_message_listener.h"
 #include "extensions/test/test_extension_dir.h"
 #include "google_apis/gaia/gaia_switches.h"
 #include "net/dns/mock_host_resolver.h"
@@ -613,16 +614,8 @@ IN_PROC_BROWSER_TEST_F(ChromeNavigationBrowserTest,
 
 // Test for https://crbug.com/866549#c2. It verifies that about:blank does not
 // commit in the error page process when it is redirected to.
-// Flaky on Linux. See https://crbug.com/981295.
-#if defined(OS_LINUX)
-#define MAYBE_RedirectErrorPageReloadToAboutBlank \
-  DISABLED_RedirectErrorPageReloadToAboutBlank
-#else
-#define MAYBE_RedirectErrorPageReloadToAboutBlank \
-  RedirectErrorPageReloadToAboutBlank
-#endif
 IN_PROC_BROWSER_TEST_F(ChromeNavigationBrowserTest,
-                       MAYBE_RedirectErrorPageReloadToAboutBlank) {
+                       RedirectErrorPageReloadToAboutBlank) {
   content::WebContents* web_contents =
       browser()->tab_strip_model()->GetActiveWebContents();
   GURL url(embedded_test_server()->GetURL("a.com", "/title1.html"));
@@ -658,10 +651,16 @@ IN_PROC_BROWSER_TEST_F(ChromeNavigationBrowserTest,
           console.log("onBeforeRequest: ", d);
           return {redirectUrl:"about:blank"};
         }, {urls: ["*://a.com/*"]}, ["blocking"]);
+        chrome.test.sendMessage('ready');
       )");
+
+  ExtensionTestMessageListener ready_listener("ready", false /* will_reply */);
   extensions::ChromeTestExtensionLoader extension_loader(browser()->profile());
   extension_loader.LoadExtension(test_extension_dir.UnpackedPath());
 
+  // Wait for the background page to load.
+  ASSERT_TRUE(ready_listener.WaitUntilSatisfied());
+
   // Remove the interceptor to allow a reload to succeed, which the extension
   // will intercept and redirect. The navigation should complete successfully
   // and commit in a process that is different than the error page one.