Revert "`span`-ify `mojo/.../data_pipe.h`: Migrate `content/test`."

This reverts commit 22e72678.

Reason for revert: Complication failure:
https://ci.chromium.org/ui/p/chromium/builders/ci/Network%20Service%20Linux/81725/overview

Original change's description:
> `span`-ify `mojo/.../data_pipe.h`: Migrate `content/test`.
>
> `span`-ification: `mojo::DataPipe[Consumer|Producer]Handle`: Migrating
> callers.  This CL `span`-ifies the callers of the APIs in
> `mojo/public/cpp/system/data_pipe.h`:
>
> * `BeginReadData` and `BeginWriteData` return (via an out parameter)
>   a `span` instead of returning a ptr+size pair.
> * `ReadData` and `WriteData` consume a `span` instead of consuming
>   a ptr+size pair.
>
> For more details see the design note here:
> https://docs.google.com/document/d/1c4NKpXwpQ9MKK1SbJ4C6MvhXI8-KJZ4jq7N4VHTHJoI/edit?usp=sharing
>
> This CL was uploaded by git cl split.
>
> R=alexmos@chromium.org
>
> Bug: 40284755
> Change-Id: I641556bb21113d1a41361fe3d09a73a0d8b728e6
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5619270


> Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
> Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
> Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1313171}

Bug: 40284755
Change-Id: I88c715eac6e7dd2316f4fc4223a1cb3e69ff6e4a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5619019
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Owners-Override: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313186}