Make handle getters return a Slot<{MessagePipe}Handle> instead.

R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/104433007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239394 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239394