Add metadata to classes that subclass HoverButton

Bug: 1159562
Change-Id: I9315a4a78f002ebc989cd76c3169b7c421fa05cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2657847


Commit-Queue: Elaine Chien <elainechien@chromium.org>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#848817}