[Text Contrast] Adding lock to g_pixel_geometry

I had incorrectly assumed that the tsan_suppression.cc entry was no
longer necessary (see
https://chromium-review.googlesource.com/c/chromium/src/+/5328608),
but at least I was able to get a repro.

This change fixes the underlying data race with a lock.

Bug: 41080383
Change-Id: I60437510553924dea9dc89140defef17751fe46c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5328814
Commit-Queue: Kurt Catti-Schmidt <kschmi@microsoft.com>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1266757}