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This CL combines AutocompleteControllerDelegate and
OmniboxControllerEmitter::Observer classes into a single
AutocompleteController::Observer class.

This is because these two classes are essentially the same, both
observing the AutocompleteController.

In the future, we can also make OmniboxControllerEmitter itself an
instance of AutocompleteController::Observer, because it too, is
essentially an observer class that's a KeyedService.

The ultimate goal is to hook up the Android omnibox and NTP realbox
to the chrome://omnibox debug surfaces.

This refactor is to pave the way for that. See the bug for the full
engineering plan on how to get there.

Bug: 1058486
Change-Id: Ieb65cc2570a5af1ba417be6a87f4779bbe4710ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088223Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751822}