[iOS] Add IOS.System.PreferredSystemFontSize

Add `IOS.System.PreferredSystemFontSize` histogram to record the
preferred system font size.
The metric is recorded once at cold start.

Fixed: 426108608
Change-Id: I9aa40a9083225c6e65fe88388a5f06b6822a2b0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6651543
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Auto-Submit: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1477186}