Moves the update functionality out of update_depot_tools into src/ in https://codereview.chromium.org/175573004 .

get_toolchain_if_required.py now expects a list of hashes on the
command line, and makes sure that it gets one of those.

toolchain2013.py saves a .json which contains information relevant to the
interests of the caller, so that it can set up the parent environment. This
is returned via the --output-json command line argument to get_...py

R=iannucci@chromium.org
BUG=323300

Review URL: https://codereview.chromium.org/168603004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@252725 0039d316-1c4b-4281-b951-d872f2087c98