[Identifiability Study] Report font existence by unique or family name

We have several ways of reporting fonts as Identifiable Surfaces.
However, we do not currently have an easy way of reporting which fonts
are loaded by the page via CSS font-family
(kLocalFontLookupByUniqueOrFamilyName is the closest thing we have,
but it has the problem that the surface key is a hash of the whole
font description, so loading the same font in several sizes causes
different keys to be reported).

Let's add a simple surface for this.

Change-Id: If9a9b1cb423e8cca4b3f7dc6d57aa4d425ed1cbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4542965


Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Antonio Sartori <antoniosartori@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147664}