add mutex to GrTextBlobCache

This should be the bulk of the work. The next part is getting it to live
in a common place, and wiring up the callbacks properly.

The callback points are fPurgeMore and fMessageBusID.

Change-Id: Ibabe285ace8a0b6531572b755a6c4f7bd41b1f6c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/298400
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
3 files changed