GitLab

This explains more what is unsafe about this mojom, and add additional
comments explaining when it is okay to use this and discouraging
general use.

R=darin@chromium.org, dcheng@chromium.org

Bug: 1144462
Change-Id: I735da27d649394f2e196d99f6b63444e370e9586
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2545203Reviewed-by: Darin Fisher <darin@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828755}