Mark http/tests/navigation/same-and-different-back.html as flaky on all platforms

TBR=japhet

BUG=355274

Review URL: https://codereview.chromium.org/208393009

git-svn-id: svn://svn.chromium.org/blink/trunk@169806 bbb929c8-8fbe-4397-9dbb-9b2b20218538