Consistently pin all dependencies - including deps_os and ones
with shortened shas.

We add --process-all-deps switch so that users can easily
tell gclient to check out all affected dependencies locally.

Bug: 570091
Change-Id: If68db98000c569ae35dd7d0a4b695eb80a589213
Reviewed-on: https://chromium-review.googlesource.com/617224Reviewed-by: Michael Moss <mmoss@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>