Add GetSharedMemoryAsSpan() to avoid unsafe buffer usage

This CL adds CommonDecoder::GetSharedMemoryAsSpan() and
Buffer::GetSpanData() to return base::span<uint8_t> instead of raw
pointers, and removes UNSAFE_TODO markers.

Bug: 40284755
Change-Id: Iada475a7855330e9dcf8010b220d2f707fe0af48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7225876
Commit-Queue: kelsen liu <[email protected]>
Reviewed-by: Kyle Charbonneau <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1558408}
NOKEYCHECK=True
GitOrigin-RevId: ecbf337873a8b19a21cea2510f2ec25dfd07bf91
5 files changed