[M148] [Pipewire] Fix mouse cursor data race.

Original change's description:
> [Pipewire] Fix mouse cursor data race.
>
> This addresses a potential data race when the mouse cursor is updated
> on the Pipewire thread and read by the capture thread.
>
> Bug: chromium:504551032
> Change-Id: I1afb9febe8bb41ce62c63872e2cb5514e908dd38
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/465440
> Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
> Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
> Auto-Submit: Mark Foltz <mfoltz@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#47502}

(cherry picked from commit 90b22181ec922900e26de71463b8e0ed47a5c2dd)

Bug: chromium:505447260,chromium:504551032
Change-Id: I1afb9febe8bb41ce62c63872e2cb5514e908dd38
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/468140
Commit-Queue: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Chrome Cherry Picker <chrome-cherry-picker@chops-service-accounts.iam.gserviceaccount.com>
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/branch-heads/7778@{#6}
Cr-Branched-From: ca896b7ffef011bbf6957c99d413c5aac602c99f-refs/heads/main@{#47319}

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 d611ca0..1aa8be5 100644
--- a/modules/desktop_capture/linux/wayland/shared_screencast_stream.cc
+++ b/modules/desktop_capture/linux/wayland/shared_screencast_stream.cc
@@ -128,8 +128,9 @@
   Mutex latest_frame_lock_ RTC_ACQUIRED_AFTER(queue_lock_);
   SharedDesktopFrame* latest_available_frame_
       RTC_GUARDED_BY(&latest_frame_lock_) = nullptr;
-  std::unique_ptr<MouseCursor> mouse_cursor_;
-  DesktopVector mouse_cursor_position_ = DesktopVector(-1, -1);
+  std::unique_ptr<MouseCursor> mouse_cursor_ RTC_GUARDED_BY(&latest_frame_lock_);
+  DesktopVector mouse_cursor_position_ RTC_GUARDED_BY(&latest_frame_lock_) =
+      DesktopVector(-1, -1);
 
   int64_t modifier_;
   std::unique_ptr<EglDmaBuf> egl_dmabuf_;
@@ -695,6 +696,7 @@
 }
 
 std::unique_ptr<MouseCursor> SharedScreenCastStreamPrivate::CaptureCursor() {
+  MutexLock latest_frame_lock(&latest_frame_lock_);
   if (!mouse_cursor_) {
     return nullptr;
   }
@@ -703,6 +705,7 @@
 }
 
 DesktopVector SharedScreenCastStreamPrivate::CaptureCursorPosition() {
+  MutexLock latest_frame_lock(&latest_frame_lock_);
   return mouse_cursor_position_;
 }
 
@@ -774,20 +777,28 @@
           mouse_frame->CopyPixelsFrom(
               bitmap_data, bitmap->stride,
               DesktopRect::MakeWH(bitmap->size.width, bitmap->size.height));
-          mouse_cursor_ = std::make_unique<MouseCursor>(
-              mouse_frame, DesktopVector(cursor->hotspot.x, cursor->hotspot.y));
+          {
+            MutexLock latest_frame_lock(&latest_frame_lock_);
+            mouse_cursor_ = std::make_unique<MouseCursor>(
+                mouse_frame,
+                DesktopVector(cursor->hotspot.x, cursor->hotspot.y));
+          }
 
           if (observer_) {
             observer_->OnCursorShapeChanged();
           }
         }
-        mouse_cursor_position_.set(cursor->position.x, cursor->position.y);
+        {
+          MutexLock latest_frame_lock(&latest_frame_lock_);
+          mouse_cursor_position_.set(cursor->position.x, cursor->position.y);
+        }
 
         if (observer_) {
           observer_->OnCursorPositionChanged();
         }
       } else {
         // Indicate an invalid cursor
+        MutexLock latest_frame_lock(&latest_frame_lock_);
         mouse_cursor_position_.set(-1, -1);
       }
     }