Mark compositor-touch-hit-rects-scroll.html flaky.

BUG=270266
TEST=none
TBR=fmalita

Review URL: https://codereview.chromium.org/22313012

git-svn-id: svn://svn.chromium.org/blink/trunk@155795 bbb929c8-8fbe-4397-9dbb-9b2b20218538