It now adds a link to the *.proto in generated files.

R=tandrii@chromium.org
BUG=953405

Change-Id: I25b53fb107611e3a09ec8cc2cb23a08b35de773f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1577700Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>