Also verify source_filename properly.

R=dpranke@chromium.org
BUG=
TEST=


Review URL: http://codereview.chromium.org/7849015

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@100205 0039d316-1c4b-4281-b951-d872f2087c98