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Design doc: go/mustash
Design doc: go/move-applist

AppList consists of two parts: Apps and Search. Everything is currently
initiated in Chrome. This commit does a few things:

- initiates all AppList UI in Ash;
- removes all AppList UI from Chrome;
- bridges Ash and Chrome with AppListController and AppListClient in
  //ash/public/interfaces/app_list.mojom;
- modifies all affected tests to apply to the new AppList architecture;
- cleans up old AppList stuff, e.g. AppList presenter mojo interfaces.

With this commit we're expecting:

- all AppList models, views are initiated in Ash;
- all other AppList logic, e.g. sync and search, still live in Chrome
  since they depend on user profile data;
- in Mus+Ash mode, where Ash and Chrome are two different processes, we
  have Search disabled;
- in classic mode, where Ash and Chrome are in the same process, we get
  SearchModel from Ash in Chrome and have Search enabled.

Bug: 733662
Change-Id: I0e1e04e832ba74f8328af40deabf930b53d70bdd
Reviewed-on: https://chromium-review.googlesource.com/954355Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Jiaquan He <hejq@google.com>
Cr-Commit-Position: refs/heads/master@{#542310}