This is a reland of 7f4c905f

Original change's description:
> gclient: Add commands to edit dependencies and variables in DEPS
> 
> 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>

Bug: 760633
Change-Id: Ia46c74d02e5cc3b67517dfa248f597cb3d98ef3d
Reviewed-on: https://chromium-review.googlesource.com/969457
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>