Reland "Allow shared memory to be mapped at arbitrary offsets."

This is a reland of commit 81cfbd974c3972824f84cc0a77a501434a92c515

The memory dump infrastructure has been updated to account for memory
addresses that are not non-aligned when MapAt() is called with an offset
that is not aligned.

Original change's description:
> Allow shared memory to be mapped at arbitrary offsets.
>
> Requiring page alignment is an implementation detail that is not
> necessary to expose in the API. Exposing it in the API has resulted in
> several consumers implementing their own workarounds to handle this,
> e.g. media::UnalignedSharedMemory, Mojo's shared buffer handling, and
> Blink's array buffer mapping code.
>
> Change-Id: I175a4a7a7eab40969d72a38a1963a72134aa86c4
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3625893
> Reviewed-by: Robert Sesek <rsesek@chromium.org>
> Commit-Queue: Daniel Cheng <dcheng@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1000628}

Change-Id: I0de4594bdf0a3e07ecab62acb890b7de1042d69f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3632844
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1001226}