Mojo: Add Dispatcher::{DuplicateBufferHandle,MapBuffer} methods.

We'll need them to implement shared memory.

(Also add TODOs for a bug that I spotted.)

R=darin@chromium.org

Review URL: https://codereview.chromium.org/169463012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252827 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252827