Use .DEPS.git instead of DEPS

This commit is allow developers get GIT version of dependencies.

Most of dependencies in <release_dir>/DEPS are pointing to subversion.
Unfortunately, svn command behind firewall randomly fails with error
"501 Not Implemented". Instead, git repos are more stable to fetch.

Without XWALK_USE_GIT set, the dependencies repo will keep use the
subversion DEPS file as usual.

This commit is depend on CL https://codereview.chromium.org/317783008

How to use
==========
$ export XWALK_USE_GIT=1
$ gclient sync
Use SvnCheckout from depot_tools

This change depend on CL https://codereview.chromium.org/321483006