[ntp][side-panel] Add metric that tracks Chrome Web Store opens

Adds metric that tracks Chrome Web Store opens in Panorama to better understand the impact of Panorama on third party theme installs.

Change-Id: I8d634fd25ba3eaf91400f0c051b1b75c052a13df
Bug: 1446720
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4545691


Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Paul Adedeji <pauladedeji@google.com>
Cr-Commit-Position: refs/heads/main@{#1146686}