GitLab

Adds:

1. "enable_typescript_bindings" gn arg, which must be set to true
   to test Typescript bindings.
2. "use_typescript_sources" gn variable that can be set to opt-in a
   target into generating Typescript bindings.
3. Test interfaces folder where interfaces will be added as features
   are implemented.
4. A mojom gn target that includes the tests interfaces. This target is
   included as part of mojo_unittests
5. Basic template and generator implementation.
6. Script to compile generated Typescript bindings into JS bindings.

Bug: 1002798
Change-Id: Id8424d707ad4b2ec9a773b291fd4a41332623274
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1817860Reviewed-by: Dan Beam <dbeam@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699993}