The previously passed branch:HEAD notation is understood by bot_update
for the main project, but not by gclient for deps'ed projects.

We don't need the colon notation at all as passing a branch implies
using the HEAD of that branch.

Bug: 740456
Change-Id: I95eb88f0de2e06bee8a3e7db24c4ad85cdb76d3e
Reviewed-on: https://chromium-review.googlesource.com/566866Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>