Fix bug with canvas size in PaintProfilerView.

Review URL: https://codereview.chromium.org/345343003

Review URL: https://codereview.chromium.org/345343003

git-svn-id: svn://svn.chromium.org/blink/trunk@176854 bbb929c8-8fbe-4397-9dbb-9b2b20218538