Unofficial gardening. Marking virtual/gpu/fast/canvas/webgl/read-pixels-test.html as Slow to see if it'll pass on Win Debug.

TBR=apavlov,eseidel
BUG=

Review URL: https://codereview.chromium.org/18170004

git-svn-id: svn://svn.chromium.org/blink/trunk@154110 bbb929c8-8fbe-4397-9dbb-9b2b20218538