Doing something like basename when concatenating base_url with
relative url.

Added use_relative_urls2 to enable switching between old
implementation and new implementation. Once the old implementation
is deprecated we can remove the use_relative_urls2.

Review URL: http://codereview.chromium.org/500015

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@34563 0039d316-1c4b-4281-b951-d872f2087c98