Reuse DataDecoder for all source registrations in a storage partition

We previously used isolated DataDecoders, but on some platforms this
could require expensive process creation. Reusing the decoder across
all source registrations in a storage partition retains the security
properties of isolating the decoding of untrusted data to a separate
process, but avoids the per-registration cost of process creation.

There seems to be no privacy or security benefit to isolating the
individual decodes from each other, as the source-registration data is
not sensitive: This is specified by ARA users and expected to be sent to
all Chrome clients as "public" information. Likewise, registrations that
are decoded in the renderer are not isolated from each other in that
respect.

Bug: 1430046
Change-Id: Id6e3c65b5d8529bbfeec5ddf63925658de4801d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4395167
Quick-Run: Andrew Paseltiner <apaseltiner@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Commit-Queue: Andrew Paseltiner <apaseltiner@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1126116}