Add an indicator UMA metric for render-blocking fonts

This will help us carry out a finch experiment on the effectiveness
of making font preloads render-blocking:

https://docs.google.com/document/d/11xC1yC735bx2b_ybnLAUa4DeAPClt_v7EZktJw7KnFo/edit?resourcekey=0-DCQxDfZyJgNvmBnBHt247w#

Bug: 1412861
Change-Id: I1cf5178e03fda4f3579e5592db447a7042a1a0b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4262908
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1107569}