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}

modules/desktop_capture/linux/wayland/shared_screencast_stream.cc

diff --git a/modules/desktop_capture/linux/wayland/shared_screencast_stream.cc b/modules/desktop_capture/linux/wayland/shared_screencast_stream.cc
index 44eab8f..d611ca0 100644
--- a/modules/desktop_capture/linux/wayland/shared_screencast_stream.cc
+++ b/modules/desktop_capture/linux/wayland/shared_screencast_stream.cc
@@ -654,13 +654,13 @@
     pw_stream_ = nullptr;
 
     {
-      MutexLock lock(&queue_lock_);
-      queue_.Reset();
-    }
-    {
       MutexLock latest_frame_lock(&latest_frame_lock_);
       latest_available_frame_ = nullptr;
     }
+    {
+      MutexLock lock(&queue_lock_);
+      queue_.Reset();
+    }
   }
 
   if (pw_core_) {
@@ -681,7 +681,7 @@
 SharedScreenCastStreamPrivate::CaptureFrame() {
   MutexLock latest_frame_lock(&latest_frame_lock_);
 
-  if (!pw_stream_ || !latest_available_frame_) {
+  if (!latest_available_frame_) {
     return std::unique_ptr<SharedDesktopFrame>{};
   }