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This CL adds a new Mojo API for observing changes to search
availability. It will be used by the search UI so that the UI updates
itself when its results become stale and no longer usable.

Bug: 1096867
Change-Id: I89e77d01d86d6d55aa69548f267dfb88f8d0090a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2261191Reviewed-by: Regan Hsu <hsuregan@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Auto-Submit: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781899}