The PostUploadHooks in the Chromium repository which add
CQ_INCLUDE_TRYBOTS entries to the issue description will be rewritten
in terms of this primitive, which will compose properly if multiple
sub-directories attempt to modify it.

BUG=688765

Change-Id: Icf72edb872f29af1e082038e96bc547504edfd07
Reviewed-on: https://chromium-review.googlesource.com/438925Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>