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Adds a new NavigableContentsView type to the Content Service's
client library. This is meant to be a platform-agnostic encapsulation
of all business relevant to the display of renderered web contents (i.e.
a navigated NavigableContents object) within a client application's UI.

This also enables NavigableContentsView embedding on all desktop Aura
platforms in addition to Chrome OS, which is the only supported platform
before this CL.

For non-Mash non-Chrome OS builds, NavigableContentsView usage is
restricted to the browser process, where Content Service instances
are hosted. Support for remote NavigableContentsViews will expand as
support for the UI service expands.

For testing, this enables --launch-in-process-simple-browser in all
desktop Aura developer builds, running an in-process Simple Browser
application which consumes NavigableContentsView to display web contents
via the Content Service. The out-of-process version
(--launch-simple-browser) still works on Chrome OS with Mash enabled.

Follow-up patches will enable local NavigableContentsView usage on
non-Aura platforms (e.g. Android, Mac, etc) so that we can start
converting WebContents uses to NavigableContents uses.

Bug: 855092
Test: Run chrome --launch-in-process-simple-browser (with DCHECKS enabled) on Chrome OS, Linux or Windows and watch it go!
Change-Id: Id091017a1656f078ea87fa3a9d6a6f4049fb4f13
Reviewed-on: https://chromium-review.googlesource.com/1171668Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583166}