viz: win: Do not wrap DCLayerOverlayParams in a unique_ptr

This CL makes it so we use DCLayerOverlayParams directly by value inside
DCLayerTree, rather than wrapping it in a std::unique_ptr. This brings
us more in line with other platforms and opens up the possibility for
more shared code in the OS compositor space.

This CL consists of the change in ui/gl/dc_layer_tree.h, followed by
changes to other files to fix the build.

It also makes a minor change of not adding null mailboxes to
`scheduled_overlay_mailboxes_`. This should have no behavioral effect
since we previously only compared `scheduled_overlay_mailboxes_`
against `overlays_`, which does not contain null mailboxes.

Change-Id: I9694dcbd3a90fc56e4f80346cff1bc4de62255cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6194781
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Michael Tang <tangm@microsoft.com>
Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1415845}