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Track number of unique search queries issued. Will be used to calculate overall Search CTR.
A unique search query is defined as whenever the search box goes from empty to non-empty.

TBR=holte@chromium.org

Bug: 1035529
Change-Id: Iaeadb897c5ad86ba6a65c9b49edb4ff8fb427d4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1977187
Commit-Queue: Jenny Zhang <jennyz@chromium.org>
Reviewed-by: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729549}