Update getAdvances for using a mutex

I'm hoping to have a few pinch points that will actually hold a mutex.
The call getAdvances uses to get glyphs, called metrics(), is one of them.

+ Update some call sites to use the new glyph() call.

Change-Id: I04158e200db43758fe6d33337e5d1137f197a5b7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/221777
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
3 files changed