[//cc] Fix HudSoftwareBacking usage of ClientSI::ScopedMapping

The code as-is unmaps the mapping inline when the ScopedMapping is
destroyed and then uses the pixels from the mapping. This isn't actually
a problem in the current implementation used, but it's a conceptual
violation of the API and liable to produce bustage later if/when the
implementation is refactored.

Bug: 40064122
Change-Id: Iec3a7712b95b06d8d9c8fe5859e37c6607197d25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6172218
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1406041}