Break apart mojom_jinja_generator back into C++ and JS versions.

I combined the 2 separate ones when I switched the C++ generator to jinja, but
it turns out they don't actually share much code. The code they do share, I
moved to the parent class.

R=darin@chromium.org

Review URL: https://codereview.chromium.org/137643002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245030 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245030