Speculative fix for a hang in `GetEmojiFont`

crrev.com/c/5622559 changed one-time initialization by
`static bool` to `std::once()`, and this looks like causing a
hang when called from `DEFINE_THREAD_SAFE_STATIC_LOCAL`. This
patch reverts to using `static bool`.

It may run the initialization code multiple times on race
conditions, but doing so should have no harm other than wasted
CPU cycles.

This is a speculative fix.

Bug: 349456407
Change-Id: I794cec88f6899bf99f41d719d8b2013843ebbd45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5776142
Reviewed-by: Kent Tamura <tkent@chromium.org>
Auto-Submit: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1339516}