Mojo: introduce bindings/lib/router.{h,cc}

Adds the plumbing necessary to support the request/response pattern for Mojo C++ bindings.

R=davemoore@chromium.org, viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/186393003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256442 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256442