mojo-ify plugin placeholder renderer->host messages

This CL introduces PluginHost, PluginAuthHost and PluginRenderer
mojo interfaces. 

PluginHost and PluginAuthHost are for messages sent
to the host.
PluginRenderer is for messages sent from host to renderer.

We pass PluginRenderer interfaces to the host when the renderer needs
new plugins to be downloaded.


Bug: 577685
Change-Id: Iabf90d2f87703f5d84506edef7d9a27d90022735
Reviewed-on: https://chromium-review.googlesource.com/627965
Commit-Queue: Nicholas Verne <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Reviewed-by: Sam McNally <[email protected]>
Reviewed-by: Nico Weber <[email protected]>
Cr-Commit-Position: refs/heads/master@{#500213}
11 files changed