[canvaskit] Direct GrContext methods to appropriate WebGL context

Change-Id: I13da784500422f3d3faedb6d9116487adf4926e0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/589403
Reviewed-by: Arman Uguray <armansito@google.com>
5 files changed