Use text size on Mac.

The current code assumes that text on Mac is freely scalable,
or at least that the text size is just a part of the transform.
However, it appears that application of the 'trak' table,
as well as other optical adjustments, may rely on the text size
directly.

This change passes the text size requested directly to CoreText.

BUG=chromium:427528

Review URL: https://codereview.chromium.org/752183002

Cherry-pick: be2284de556e7ec29489693093d42dc86b762645
Approval: https://code.google.com/p/chromium/issues/detail?id=427528#c24