Unofficial gardening. Mark some fast/canvas and fast/canvas/webgl tests as Slow.

BUG=24182
TBR=vsevik

Review URL: https://codereview.chromium.org/18801002

git-svn-id: svn://svn.chromium.org/blink/trunk@153646 bbb929c8-8fbe-4397-9dbb-9b2b20218538