Handling plugins externally for MimeHandlerView

This CL routes the call IsPluginHandledExternally() to the extensions
layer.

The work here will soon be followed up by followup CLs which implement
the logic MimeHandlerViewGuest based on cross-process frames.

PoC CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1121680

Design Doc:
https://docs.google.com/document/d/10g7Y9cprYKkch9JZ0TBUWaEnHBJT1nzhskQIt1nHbWM/edit#heading=h.ue5a8s290yhk

Bug: 659750
Change-Id: If8546f80842c726fa95aba702f35e6d594fc09e8
Reviewed-on: https://chromium-review.googlesource.com/1161115
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581440}
11 files changed