Isolate updating grTextStrike

I'm isolating this code into a single routine, so I can change it
over to using a bulk API from the strike. The goal is to remove
all uses of bulk metrics and images.

Change-Id: I8c1c9a4e619962c8e35084c026a4c4553fbeb577
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/264837
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed