Add builder support to MatchCountLabel

* Rename MatchCountLabel to FindBarMatchCountLabel and move it
  to top file level, because builder does not support inner class.
* Add builder support to it.

Bug: 1130078
Change-Id: I6ada19042466ca1216c60858f5542d17cf0c1626
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2441491
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Auto-Submit: Keren Zhu <kerenzhu@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814785}