Mark virtual/gpu/fast/canvas/canvas-composite-text-alpha.html as slow
Mark virtual/gpu/fast/canvas/canvas-composite-text-alpha.html as slow

TBR=junov@chromium.org
BUG=

Review URL: https://codereview.chromium.org/238983008

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@171806 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs