cc: Make OutputSurfaceFrame::sub_buffer_rect optional

cc/output/output_surface_frame.h

Index: cc/output/output_surface_frame.h
diff --git a/cc/output/output_surface_frame.h b/cc/output/output_surface_frame.h
index 2306f2e5b4bbe686807d324dd438eee65266d245..331496fe06ebfb7b385e259098125fce7cf49943 100644
--- a/cc/output/output_surface_frame.h
+++ b/cc/output/output_surface_frame.h
@@ -8,6 +8,7 @@
 #include <vector>
 
 #include "base/macros.h"
+#include "base/optional.h"
 #include "cc/base/cc_export.h"
 #include "ui/events/latency_info.h"
 #include "ui/gfx/geometry/rect.h"
@@ -26,7 +27,8 @@ class CC_EXPORT OutputSurfaceFrame {
   OutputSurfaceFrame& operator=(OutputSurfaceFrame&& other);
 
   gfx::Size size;
-  gfx::Rect sub_buffer_rect;
+  // Optional rect for partial or empty swap; if not provided, use regular swap.
+  base::Optional<gfx::Rect> sub_buffer_rect;
   std::vector<ui::LatencyInfo> latency_info;
 
  private: