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

This reverts commit 2df162db and
therefore relands https://crrev.com/c/5619270.  This is because the
prerequisite CL has been relanded in https://crrev.com/c/5622694.
The original CL description follows below.

`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: I968760300f1855a5dbdc5bf25e92686c190c06d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5638262


Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1316548}