Move using declarations to the anonymous namespace

From https://abseil.io/tips/119:
Declare namespace aliases and using-declarations inside the innermost
namespace, whether named or anonymous. (Do not add an anonymous
namespace just for this purpose.)”

Bug: 413314866
Change-Id: I3a6e6c0e32f6bc69e1088f8f47306a92b5a71f40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6605893
Reviewed-by: Olivier Li <olivierli@chromium.org>
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1468723}