[Ozone-Drm] Pick pending buffer over current buffer when modesetting

ui/ozone/platform/drm/gpu/hardware_display_controller.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_OZONE_PLATFORM_DRM_GPU_HARDWARE_DISPLAY_CONTROLLER_H_
 #define UI_OZONE_PLATFORM_DRM_GPU_HARDWARE_DISPLAY_CONTROLLER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 #include <xf86drmMode.h>
@@ skipping 162 matching lines @@
     ~PageFlipRequest();
 
     OverlayPlaneList planes;
     bool is_sync;
     base::Closure callback;
   };
 
   // Buffers need to be declared first so that they are destroyed last. Needed
   // since the controllers may reference the buffers.
   OverlayPlaneList current_planes_;
+  // Planes currently being queued without having SchedulePageFlip() called.
   OverlayPlaneList pending_planes_;
+  // Plane lists for which SchedulePageFlip() was called. They are waiting for
+  // previous page flip events to be completed before they can be serviced.
   std::deque<PageFlipRequest> requests_;
   scoped_refptr<ScanoutBuffer> cursor_buffer_;
 
   base::ScopedPtrHashMap<DrmDevice*, HardwareDisplayPlaneList>
       owned_hardware_planes_;
 
   // Stores the CRTC configuration. This is used to identify monitors and
   // configure them.
   ScopedVector<CrtcController> crtc_controllers_;
 
   // Location of the controller on the screen.
   gfx::Point origin_;
 
   // The mode used by the last modesetting operation.
   drmModeModeInfo mode_;
 
   bool is_disabled_;
 
   DISALLOW_COPY_AND_ASSIGN(HardwareDisplayController);
 };
 
 }  // namespace ui
 
 #endif  // UI_OZONE_PLATFORM_DRM_GPU_HARDWARE_DISPLAY_CONTROLLER_H_