Cleaning up FontCustomPlatformData.

Mostly code movement. Split the Skia and Windows logic into separate files
(since they had almost nothing in common anyhow), unify the headers, and update
use of smart pointers.

BUG=None
R=pdr,fmalita

Review URL: https://chromiumcodereview.appspot.com/20049004

git-svn-id: svn://svn.chromium.org/blink/trunk@155449 bbb929c8-8fbe-4397-9dbb-9b2b20218538