Ensure that font size precision matches font-cache precision

Ensure that the precision of the effective font size matches the
precision of the font size in the font-cache key. This guarantees
that the same precision is used regardless of cache status.

This in turn fixes a bunch of flaky svg text tests and allow us to
re-enable subpixel font scaling on Mac.

BUG=324553
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/100393009

git-svn-id: svn://svn.chromium.org/blink/trunk@163145 bbb929c8-8fbe-4397-9dbb-9b2b20218538