[layout-unit] Ensure single copy of global variables

This patch ensures that there is only one copy of global
variables defined in `layout_unit.h` by using C++17 inline
variables.

See https://abseil.io/tips/168 for details.

This patch has no behavior changes.

Bug: 351092748
Change-Id: Idbdfabcfe3e3e34f9de46aac81f559ab6cdd7924
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5678635
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Auto-Submit: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1323487}