Add build arg for ui/base ime dependency

Building ui/base/ IME code on chromeos currently implies pulling in
dependencies on third_party/gtk+ and //chromeos and a number of other
targets that don't exist in the mojo repo. We don't need this bit of
IME code for chromeos right now, so add a flag to compile it out.

R=sky@chromium.org

Review URL: https://codereview.chromium.org/672043002