`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}