Revert "Remove GrContext"

This reverts commit 2edf18d818708509809f4be17a2f5abd6d93af2b.

Reason for revert: breaking wasm-debugger

Original change's description:
> Remove GrContext
>
> Woo!
>
> Change-Id: Ifa3ff7e79c7048c2f1d808cc9705593d72886f08
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/326436
> Auto-Submit: Adlai Holler <adlai@google.com>
> Reviewed-by: Robert Phillips <robertphillips@google.com>
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Robert Phillips <robertphillips@google.com>
> Commit-Queue: Brian Salomon <bsalomon@google.com>

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

Change-Id: Ie05bda5dad7fb5d369ebe579d2e009fdb99c0729
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/326941
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
29 files changed