Fix bug where setting canvas size to 0 was treated as if context was lost

The canvas context lost handling logic was falsely kicking, rendering
the canvas unusable when setting one of its dimensions to zero.

BUG=532148

Review URL: https://codereview.chromium.org/1350883002

git-svn-id: svn://svn.chromium.org/blink/trunk@202645 bbb929c8-8fbe-4397-9dbb-9b2b20218538