Mark vertical-scrollbar-in-rtl.html as flaky after rebaseline in r154935

BUG=231910
TBR=ojan

Review URL: https://codereview.chromium.org/20503002

git-svn-id: svn://svn.chromium.org/blink/trunk@154937 bbb929c8-8fbe-4397-9dbb-9b2b20218538