make GrTextBlobCache::remove thread safe

Change-Id: I48979516c13fcfedcb9580e05c899d800553d55a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/375072
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Adlai Holler <adlai@google.com>
1 file changed