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This is the main controller for the Bloom feature.

In this CL I simply:
   * Introduce the |BloomController| class, and an (mostly) empty
    |BloomControllerImpl|.
   * Create the |BloomController| inside the |AssistantClient|, by means
    of a |BloomControllerFactory|.

In later CLs this will actually start doing something.

Bug: b/165356952
Change-Id: Ib4d22016bd60ae27e940807e9bb47a2d317eaf96
Tests: Compiled
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363399Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Jeroen Dhollander <jeroendh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801390}