Don't round x-axis for text runs when subpixel font scaling is enabled

When subpixel font scaling is enabled text runs are positioned at
subpixel boundaries on the x-axis and thus there is no reason to snap
the x value.
We still round the y-axis to ensure consistent line heights.

BUG=164167
R=leviw@chromium.org, mstensho@opera.com

Review URL: https://codereview.chromium.org/300623007

git-svn-id: svn://svn.chromium.org/blink/trunk@174992 bbb929c8-8fbe-4397-9dbb-9b2b20218538