Mark virtual/gpu/fast/canvas/webgl/webgl-large-texture.html as slow

TBR=kbr@chromium.org
BUG=358313

Review URL: https://codereview.chromium.org/218623010

git-svn-id: svn://svn.chromium.org/blink/trunk@170481 bbb929c8-8fbe-4397-9dbb-9b2b20218538