GitLab

- New doc explaining how to initialize blink features
  in content layer
- Update feature flag guides to point to the new doc

Bug: 832393
Change-Id: Id0cd47a2b926c55e15516242cbf6382e199b1537
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1873991Reviewed-by: Jason Chase <chasej@chromium.org>
Commit-Queue: Rodney Ding <rodneyding@google.com>
Cr-Commit-Position: refs/heads/master@{#711828}