Mark virtual/gpu/fast/canvas/canvas-composite-stroke-alpha.html as flaky in WebKit Win Oilpan (dbg), WebKit Win7 (dbg)

This test looks flaky for a long time in [WebKit Win Oilpan (dbg), WebKit Win7 (dbg)].

I can't tell the exact possible revision from the dashboard:
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=virtual%2Fgpu%2Ffast%2Fcanvas%2Fcanvas-composite-stroke-alpha.html

TBR=leviw@chromium.org
BUG=363029

Review URL: https://codereview.chromium.org/236633005

git-svn-id: svn://svn.chromium.org/blink/trunk@171437 bbb929c8-8fbe-4397-9dbb-9b2b20218538