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Changes the app_list UI to use Content Service client library to display
answer card web contents, rather than using some one-off IPCs to drive
remote WebContents behavior with a special WebContentsDelegate/Observer
in chrome/browser code. This allows quite a bit of code to be deleted
from src/chrome while retaining all the existing functionality of answer
cards in the app_list search UI.

Prior to this change, AnswerCardSearchProvider would query the potential
card URL (i.e. google.com/coac?q=whatever) and only send a corresponding
search result back to Ash if the response was a valid card response.
Instead, now all queries are treated as potentially valid search results,
and AnswerCardSearchProvider is little more than a translator which turns
a search query string into a useful answer card query URL.

Ash-side code then navigates its own NavigableContents to any potential
card URL it receives, inspecting response headers to determine whether
or not the contents are actually a card. If they are, it displays them
in the UI.

The key results of this CL are:

* Most relevant app_list answer card support logic lives in Ash now. A
future change could probably move the rest of it, but that requires a
more fundamental reconsideration of the browser's role (or non-role) in
supporting app_list search behavior.

* All of the Ash/Chrome code to support answer cards in the app_list
UI is unified for in- and out-of-process Ash cases.

* We have a working Content Service client being used in production
Chrome OS code, which establishes precedent for improving several other
features that embed WebContents within Chrome OS system UI.

Bug: 854367
Change-Id: Id913460d7a4bd4558d385b9b3f9b52845aa0f0e7
Reviewed-on: https://chromium-review.googlesource.com/c/1269622
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Eric Roman <eroman@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605166}