Fix accidental double import in mojom_bindings_generator.py.

BUG=329008
TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/116893005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241992 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241992