This includes a minor refactor so that some gclient_scm methods
can all share the same core.quotePath specifier.

R=iannucci

Bug: 792302
Change-Id: Iaadf190f5c0666787cf7c2ccda88d6dba9aace9b
Reviewed-on: https://chromium-review.googlesource.com/823131Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>