Update Bazel files

 - Use latest emscripten toolchain (3.1.0)
 - Autogenerate the atoms and manually fix some of the file lists.
 - Add a known_good_builds target to bazel/Makefile to help
   check the things we expect to work with Bazel.

Change-Id: Ia5f51e7b9eb5c108386820ad59180c8f862f5a70
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/491438
Reviewed-by: Ben Wagner <bungeman@google.com>
39 files changed