Move guest view container message routing out of content/.
GuestViews used to receive messages through BrowserPlugin, this CL makes
GuestView message routing possible without BrowserPlugin. This is a
prerequisite for making <webview> pluginless.
BUG=
330264
Test=None, internal change only.
Committed:
https://crrev.com/7a4f1ceb18ffe6777d6a8b10b2db06e957187eba
Cr-Commit-Position: refs/heads/master@{#332430}