Mark LayoutTests virtual/gpu/fast/canvas/canvas-lose-restore-googol-size.html as flaky on Win7 (dbg)

BUG=401902
R=haraken@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/453883002

git-svn-id: svn://svn.chromium.org/blink/trunk@179809 bbb929c8-8fbe-4397-9dbb-9b2b20218538