Remove global mutex around CG

Change-Id: I3df1b4f475c3e50e3432b40aaeee220ff4acb586
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/213541
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed