Unreviewed gardening. Mark Kusa-Makura-background-canvas.html as flaky.

TBR=dglazkov@chromium.org
BUG=339174

Review URL: https://codereview.chromium.org/142413012

git-svn-id: svn://svn.chromium.org/blink/trunk@166048 bbb929c8-8fbe-4397-9dbb-9b2b20218538