Add support for enums within structs and interfaces to mojom.

BUG=320090
R=abarth@chromium.org, davemoore@chromium.org

Review URL: https://codereview.chromium.org/99623010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243341 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243341