Mojo: Add the beginnings of a proper embedder API.

Actually, just move mojo::system::CoreImpl::Init() to
mojo::embedder::Init(), in its own header file.

R=darin@chromium.org

Review URL: https://codereview.chromium.org/135073002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244284 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244284