Cache SkTypeface in FontPlatformData on Mac

Before painting text on Mac, we call setupPaint, which would
use the CoreText font to create an SkTypeface object. Instead
of holding onto this, we were promptly throwing it away. Other
platforms keep the SkTypeface around on their version of
FontPlatformData. I'm mirroring that for Mac now.

The previous version of this patch caused assertions due to a
missing adoptRef :( You can find it at
https://codereview.chromium.org/141413002/

Review URL: https://codereview.chromium.org/138903018

git-svn-id: svn://svn.chromium.org/blink/trunk@165676 bbb929c8-8fbe-4397-9dbb-9b2b20218538