mojo: break struct headers into headers and source

BUG=None
TEST=None
R=darin@chromium.org, darin

Review URL: https://codereview.chromium.org/52973003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231954 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231954