[fuchsia] Allow main thread blocking during resource reload.

When reloading locale-specific resources, allow blocking on the main
thread, since some small amount of jank during locale changes is
acceptable.

Bug: 1192341
Change-Id: I0f9a53e28af3cd6e823890ebc29c2d032589c8ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3110553


Commit-Queue: Wez <wez@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#915145}