Make DisplayConfigureRequest own DisplayMode clones.

Prior to this change, DisplayConfigureRequest would indicate a
requested mode via a pointer to a DisplayMode owned by the corresponding
DisplaySnapshot. In order to support the virtual modes feature, it is
no longer valid to assume that a requested mode already exists by the
snapshot. This change replaces DisplayMode pointers with unique_ptrs to
enforce that DisplayConfigureRequests will always own their requested
mode.

When creating a request, modes are cloned as needed (if they already
exist on the snapshot), or created anew in the virtual case. When a
request succeeds, the newly applied mode is matched against existing
modes on the snapshot, or cloned and added to the snapshot in the
virtual case.

Bug: b:321725211
Change-Id: I16ff0a25dc1caac09ae930ca5eb5f4e17c28d039
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5606024
Reviewed-by: Gil Dekel <gildekel@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Andrew Wolfers <aswolfers@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1329024}