Mark NewTabUITest.ChromeInternalLoadsNTP flaky on all platforms.

They occasionally fail on other platforms too.
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=NewTabUITest.ChromeInternalLoadsNTP&testType=ui_tests

BUG=45001
TEST=green

Review URL: http://codereview.chromium.org/2825034

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@51549 0039d316-1c4b-4281-b951-d872f2087c98