It is done in preparation for switching to OAuth2 as default (and only)
authentication method. Having all auth options handled by the same code makes it
easier to gradually add OAuth2 support.

As part of this, some options that would no longer work with OAuth2 (and that
are not being used from anywhere now, as far as I can tell) are removed:
  * Passing account password for authentication via command line.
  * Overriding 'Host' header when making requests to Rietveld (won't work with
    SSL anyway).
  * --account_type option (seems to be ClientLogin specific).

R=maruel@chromium.org
BUG=356813

Review URL: https://codereview.chromium.org/1075723002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294746 0039d316-1c4b-4281-b951-d872f2087c98