In SFI NaCl, ChannelHandle is just a file descriptor. On other platforms
it's just a mojo message pipe handle. This CL removes the other unused
fields. It also removes IPC::Channel::GenerateMojoChannelHandlePair()
and updates its remaining use.

BUG=659448

Review-Url: https://codereview.chromium.org/2484943004
Cr-Commit-Position: refs/heads/master@{#431821}