sync compositor: Set display viewport for software draw

cc/surfaces/display.h

Index: cc/surfaces/display.h
diff --git a/cc/surfaces/display.h b/cc/surfaces/display.h
index 3891121d90107e432f6ec757344da1525a601529..35ea2a86ed9e54b3eb81e17dd909a381208863d6 100644
--- a/cc/surfaces/display.h
+++ b/cc/surfaces/display.h
@@ -76,6 +76,7 @@ class CC_SURFACES_EXPORT Display : public DisplaySchedulerClient,
   void Resize(const gfx::Size& new_size);
   void SetColorSpace(const gfx::ColorSpace& color_space);
   void SetExternalClip(const gfx::Rect& clip);
+  void SetExternalViewport(const gfx::Rect& viewport);
   void SetOutputIsSecure(bool secure);
 
   const SurfaceId& CurrentSurfaceId();
@@ -133,6 +134,7 @@ class CC_SURFACES_EXPORT Display : public DisplaySchedulerClient,
   gfx::ColorSpace device_color_space_;
   bool swapped_since_resize_ = false;
   gfx::Rect external_clip_;
+  gfx::Rect external_viewport_;
   gfx::Size enlarge_texture_amount_;
   bool output_is_secure_ = false;