Gardening: Marking test virtual/gpu/fast/canvas/canvas-lost-gpu-context.html as flaky on Win

BUG=327698
TBR=senorblanco

Review URL: https://codereview.chromium.org/93933008

git-svn-id: svn://svn.chromium.org/blink/trunk@163710 bbb929c8-8fbe-4397-9dbb-9b2b20218538