Split GrContextThreadSafeProxy into its own files (take 2)

This just shuffles stuff around.

Change-Id: I90e24a73bf44dd3e88fc121a0528bf94e4fdead4
Reviewed-on: https://skia-review.googlesource.com/c/187786
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
12 files changed