Gardening: moving virtual/gpu/fast/canvas/canvas-composite-stroke-alpha.html

It was previously marked as Pass Timeout, but looking at the dashboard it
appears to just be slow on all platforms.

TBR=hayato@chromium.org
BUG=363029

Review URL: https://codereview.chromium.org/237963007

git-svn-id: svn://svn.chromium.org/blink/trunk@171713 bbb929c8-8fbe-4397-9dbb-9b2b20218538