Eliminate kMojoChannelToken usage for render processes

Changes RenderProcessHostImpl to connect its IPC Channel
by requesting a nominal IPC::mojom::ChannelBootstrap
interface from its shell::Connection to the render process.

Changes ChildThreadImpl to support binding such requests
by fusing them to its local IPC Channel if the (now deprecated)
kMojoChannelToken was not provided.

BUG=623396
[email protected]

Review-Url: https://codereview.chromium.org/2201183005
Cr-Commit-Position: refs/heads/master@{#410656}
9 files changed