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The motion spec dictates that the stage should not be cleared when a
new query is committed. Instead, the previous query response should
fade out while we wait for the next response to come in (which should
also be animated).

This CL does not implement the entire motion spec, only the portion
that fades out the previous query response while we are waiting for
the next response to arrive.

Changes are guarded behind kIsMotionSpecEnabled.

See bug for spec/demo.

Bug: b:112091713
Change-Id: I88cb4703f8b8b697ce87ac5a70162853d8826244
Reviewed-on: https://chromium-review.googlesource.com/1159223Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#580605}