Tab Search: reduce debounce timeout value

This change makes close tab more responsive.
Since TabStripModel::CloseWebContentsAt() mostly return false,
how fast TabsChanged() is sent
to the web/js side depends on the debounce timeout value.

Bug: 1099917

Change-Id: Icd7c2a5bc45ba2b8361f8ccc5d61033ed0103462
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2335505
Commit-Queue: Yuheng Huang <yuhengh@chromium.org>
Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794353}