Enable trivial ABI for CanvasKit and Bazel build

This is a follow-up to http://review.skia.org/633089

This introduces a custom platform constraint which one could
use in a platform definition. This also lets us opt-in to it
in WASM builds (since I believe those don't care about ABI
compatibility).

Change-Id: I68c498d03726e82bd62f84581be14ea12bba3f2d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/678276
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
4 files changed