split out fontmetrics into its own struct

Future CLs will migrate all callers to use SkFontMetrics,
so we can remove the SkPaint typedef.

Next migrate the world to use SkFont::getMetrics() instead

Bug: skia:2664
Change-Id: I2aa45cd88762c3d3589c12f5074974af7fb85410
Reviewed-on: https://skia-review.googlesource.com/c/168641
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Reed <reed@google.com>
6 files changed