Update css3-ch-unit test and harness to support subpixel font sizes

Update the fast/css/css3-ch-unit.html and the js-test.js test harness
to add support for tolerance comparsions which will be needed once
subpixel font scaling is enabled.

BUG=164167
R=dglazkov@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/292293002

git-svn-id: svn://svn.chromium.org/blink/trunk@174543 bbb929c8-8fbe-4397-9dbb-9b2b20218538