Wayland screencast: fix data race in CaptureFrame()

CaptureFrame() was reading pw_stream_ without holding any lock, which
races with StopAndCleanupStream() and StartScreenCastStream() accessing
it from other threads. Remove the unguarded pw_stream_ check and rely
solely on latest_available_frame_, which is already protected by
latest_frame_lock_.

Bug: webrtc:42223634
Change-Id: I8e1df44f4ec971948411be45cdf4140b2dcfc553
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/453020
Reviewed-by: Mark Foltz <mfoltz@chromium.org>
Reviewed-by: Andreas Pehrson <apehrson@mozilla.com>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Mark Foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#47092}