Add a per-file rule for font_matching_metrics.(cc|h).

Requested by drott@ in crrev.com/c/2587435.

Bug: 1025945
Change-Id: Ibcf257dabde32da874fd300b91e01d597d7d3a42
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2597458
Commit-Queue: Caleb Raitto <caraitto@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838221}

third_party/blink/renderer/platform/fonts/OWNERS

+per-file font_matching_metrics.cc=file://third_party/blink/public/common/privacy_budget/OWNERS
+per-file font_matching_metrics.h=file://third_party/blink/public/common/privacy_budget/OWNERS
\ No newline at end of file