Always use ChannelMojo for content child processes.

All content child processes now use ChannelMojo. This removes the option
to not use ChannelMojo.

BUG=604282

Review-Url: https://codereview.chromium.org/2389193004
Cr-Commit-Position: refs/heads/master@{#423375}
34 files changed