GitLab

Many spots using forward_variables_from(invoker, "*") were not
allowing these two variables to be set in outer scopes.

* Introduce a helper variable "EXPLICIT_FORWARDS" for use with the
recommended pattern of:
  forward_variables_from(invoker, "*", [ "testonly", "visibility" ])
  forward_variables_from(invoker, [ "testonly", "visibility" ])
* Ensures this pattern is used in android templates, test.gni,
  and BUILDCONFIG.gn
* Documents this pattern in writing_gn_templates.md
* Adds a PRESUBMIT.py for it
* Fixes visibility of a few blink targets now that test()
  respects it.

Bug: 862232
Change-Id: Ib71dbf34be76131fc749c721aea856e1146bc69a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2454427
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#830678}