Add selectable GPU backend to wasm debugger

Bug: skia:
Change-Id: If34e08cb320a141cb838caafeffa4f1f69c5234f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201404
Commit-Queue: Nathaniel Nifong <nifong@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
6 files changed