Fix racy NavigationBrowserTest usage of TestNavigationObserver

What:
Replace usage of ShellAddedObserver + TestNavigationObserver with
TestNavigationObserver::StartWatchingNewWebContents().

Why:
Tests are flaking from calls to TestNavigationObserver::Wait()
hanging.

The existing solution assumes that the test will setup the
TestNavigationObserver before the newly opened WebContents starts the
navigation.

However this isn't necessarily true, as we aren't registering the
TestNavigationObserver in the same callstack in which the web contents
is created.

Bug: 1166341
Change-Id: Id2b5f36158f86377b3881e54d793c27af718cb1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2627757Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Charlie Reis <creis@chromium.org>
Commit-Queue: John Delaney <johnidel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843300}

content/browser/navigation_browsertest.cc

@@ -1048,15 +1048,14 @@ IN_PROC_BROWSER_TEST_F(NavigationBrowserTest,
         &success));
     success = false;
 
-    ShellAddedObserver new_shell_observer;
+    TestNavigationObserver observer(url);
+    observer.StartWatchingNewWebContents();
     EXPECT_TRUE(ExecuteScriptAndExtractBool(
         shell(),
         "window.domAutomationController.send(clickCrossSiteNewWindowLink());",
         &success));
     EXPECT_TRUE(success);
 
-    TestNavigationObserver observer(
-        new_shell_observer.GetShell()->web_contents());
     observer.Wait();
     EXPECT_EQ(url, observer.last_navigation_url());
     EXPECT_TRUE(observer.last_navigation_succeeded());
@@ -1090,7 +1089,8 @@ IN_PROC_BROWSER_TEST_F(NavigationBrowserTest,
         &success));
     success = false;
 
-    ShellAddedObserver new_shell_observer;
+    TestNavigationObserver observer(url);
+    observer.StartWatchingNewWebContents();
     EXPECT_TRUE(
         ExecuteScriptAndExtractBool(shell(),
                                     "window.domAutomationController.send("
@@ -1098,8 +1098,6 @@ IN_PROC_BROWSER_TEST_F(NavigationBrowserTest,
                                     &success));
     EXPECT_TRUE(success);
 
-    TestNavigationObserver observer(
-        new_shell_observer.GetShell()->web_contents());
     observer.Wait();
 
     EXPECT_EQ(url, observer.last_navigation_url());
@@ -1139,15 +1137,14 @@ IN_PROC_BROWSER_TEST_F(NavigationBrowserTest,
         &success));
     success = false;
 
-    ShellAddedObserver new_shell_observer;
+    TestNavigationObserver observer(url);
+    observer.StartWatchingNewWebContents();
     EXPECT_TRUE(ExecuteScriptAndExtractBool(
         subframe_rfh,
         "window.domAutomationController.send(clickCrossSiteNewWindowLink());",
         &success));
     EXPECT_TRUE(success);
 
-    TestNavigationObserver observer(
-        new_shell_observer.GetShell()->web_contents());
     observer.Wait();
     EXPECT_EQ(url, observer.last_navigation_url());
     EXPECT_TRUE(observer.last_navigation_succeeded());
@@ -1222,14 +1219,13 @@ IN_PROC_BROWSER_TEST_F(NavigationBrowserTest,
         &success));
     success = false;
 
-    ShellAddedObserver new_shell_observer;
+    TestNavigationObserver observer(url);
+    observer.StartWatchingNewWebContents();
     EXPECT_EQ(true, EvalJs(shell(), R"(
       target = document.getElementById('cross_site_link');
       var evt = new MouseEvent("click", {"button": 1 /* middle_button */});
       target.dispatchEvent(evt);)"));
 
-    TestNavigationObserver observer(
-        new_shell_observer.GetShell()->web_contents());
     observer.Wait();
     EXPECT_EQ(url, observer.last_navigation_url());
     EXPECT_TRUE(observer.last_navigation_succeeded());