Revert "Split GrContextThreadSafeProxy into its own files"

This reverts commit f8397f2b3f00ca17da3c028a287097b5d79cd315.

Reason for revert: Breaking Flutter build!

Original change's description:
> Split GrContextThreadSafeProxy into its own files
> 
> This just shuffles stuff around.
> 
> Change-Id: Ieab35f50945efe87512d7077cb994132f0e0b6ef
> Reviewed-on: https://skia-review.googlesource.com/c/186874
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Robert Phillips <robertphillips@google.com>

TBR=bsalomon@google.com,robertphillips@google.com

Change-Id: I236add95d8a0066037854b5532a7db131d93d603
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/187783
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
12 files changed