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This GuestView type would be used to render out of process pdf through extension.
This introduces BrowserPluginDelegate in content/renderer,
because we need a way to create a MimeHandlerView guest (extensions/ concept)
from BrowserPlugin (content/ concept).

MimeHandlerView is behind --enable-mime-handler-view flag.

BUG=303491
Test=Apply http://crrev.com/392813003/ on top of this CL,
Launch chrome with --out-of-process-pdf flag and --enable-mime-handler-view flag
Navigate to a PDF
Check PDF is served using BrowserPlugin, probably easier to see process associated with the BrowserPlugin from Chrome's task manager.

Review URL: https://codereview.chromium.org/376033002

Cr-Commit-Position: refs/heads/master@{#293613}