Fix use-after-free in ScreenCast and Camera portal D-Bus callbacks

GDBus async callbacks fire on the GLib main thread with a raw pointer
to the portal object. When the portal is destroyed on another thread,
the callback accesses freed memory.

Introduce PortalGuard, a ref-counted mutex-protected wrapper that
outlives the portal. Callbacks lock the guard and check the portal
pointer before use. Stop() locks the same mutex to null the pointer,
blocking until any in-flight callback finishes. Utility functions
now take scoped_refptr<PortalGuard> and manage refs internally.

Bug: chromium:491979284
Bug: chromium:499587071
Change-Id: I80fe20c5c3b6509666554c7cc7454f09cab6c2e4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/463800
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Jan Grulich <grulja@gmail.com>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Andreas Pehrson <apehrson@mozilla.com>
Cr-Commit-Position: refs/heads/main@{#47444}