Enable a sentinel file to enable/disable "depot_tools" auto-updating.
This file, if present, indicates that automatic updating should not
occur.

Add a simple control script to enable/disable automatic updating.

BUG=chromium:727917
TEST=local
  - Ran locally, --enable, --disable.

R=iannucci@chromium.org, maruel@chromium.org, nodir@chromium.org

Change-Id: Id30a17586248228849d046f79d7e0bc445d97f97
Reviewed-on: https://chromium-review.googlesource.com/521884
Commit-Queue: Daniel Jacques <dnj@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>