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This CL adds a UMA histogram to track the API performance of the
getTabs() API for TabSearch.

Matching WebUI CL below:
https://chrome-internal-review.googlesource.com/c/chrome/browser/resources/tab_search/+/3291206

Bug: 1099917
Change-Id: I8ddfddd5197f0d0641a3dcc3bf6c86fe118c07fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2432589
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811727}