spanification: Add `#pragma allow_unsafe_buffers` to core/layout/

https://chromium.googlesource.com/chromium/src/+/main/docs/unsafe_buffers.md#suppressions

This is a preparation to fix each files.
This CL has no behavior changes.

Bug: 351564777
Change-Id: I73d4e812f7b306fc4fc94e328b4ff2b0cdd454e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5681525
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1324624}
NOKEYCHECK=True
GitOrigin-RevId: 8b8f9b2e11d9bd82e69632de0f633f80a516aa9d
1 file changed