Use .DEPS.git instead of DEPS

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.
    
Since upstream is switching SVN to GIT, it is time to switch to
 GIT(.DEPS.git) totally.
    
BUG=XWALK-1894
OK, i see. The `src` is still listed in `.DEPS.git` in chromium repo, then it truly needs the `'src': None` here.
Looking at the merged commit in chromium-crosswalk, the `src` entry in added by https://github.com/crosswalk-project/chromium-crosswalk/commit/d352a08f3f579fa0e878fb99521ab1aefde7cc48#diff-4e5eaa12c3eca8c66f0d5ca7fc8f9dd3R32 . If it's coming from upstream, I should be a bug from upstream.