Adds 'gclient setvar' and 'gclient setdep' commands to edit variables
and dependencies in a DEPS file.

Bug: 760633
Change-Id: I6c0712cc079dbbbaee6541b7eda71f4b4813b77b
Reviewed-on: https://chromium-review.googlesource.com/950405
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>