Move strike cache backpointer to glyph cache

Change-Id: Ibe53217753d164a13e5666d6444f75a51b7f9c39
Reviewed-on: https://skia-review.googlesource.com/c/170762
Commit-Queue: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed