[canvaskit] New public API methods for WebGPU context creation

Bug: skia:13427
Change-Id: I259d3617317a1473c6f703dd2023e63e42d8e534
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/550327
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Arman Uguray <armansito@google.com>
5 files changed