Disabled WebviewLoginTest.BackButton.

Flaky on Linux ChromiumOS bots.

TBR=nkostylev@chromium.org
NOTRY=true
BUG=512648

Review URL: https://codereview.chromium.org/1249783003

Cr-Commit-Position: refs/heads/master@{#339811}

chrome/browser/chromeos/login/webview_login_browsertest.cc

@@ -45,7 +45,8 @@ IN_PROC_BROWSER_TEST_F(WebviewLoginTest, Basic) {
   session_start_waiter.Wait();
 }
 
-IN_PROC_BROWSER_TEST_F(WebviewLoginTest, BackButton) {
+// Flaky: http://crbug.com/512648.
+IN_PROC_BROWSER_TEST_F(WebviewLoginTest, DISABLED_BackButton) {
   WaitForGaiaPageLoad();
 
   // Start: no back button, first page.