DesktopRect::UnionWith() to extend current rect to cover the input rect

webrtc/modules/desktop_capture/desktop_geometry.h

Index: webrtc/modules/desktop_capture/desktop_geometry.h
diff --git a/webrtc/modules/desktop_capture/desktop_geometry.h b/webrtc/modules/desktop_capture/desktop_geometry.h
index 8e9b9183a82e9f781840da14ae977ecd442ba09e..495ef715808a3815461e6930d27ee375c548c1b8 100644
--- a/webrtc/modules/desktop_capture/desktop_geometry.h
+++ b/webrtc/modules/desktop_capture/desktop_geometry.h
@@ -138,6 +138,10 @@ class DesktopRect {
               int32_t right_offset,
               int32_t bottom_offset);
 
+  // Finds the minimum area to cover both |this| and |rect|. If |this| is empty,
+  // replaces |this| with |rect|.

[jamiewalch, 2017/05/02 22:18:11]

This is not symmetric w.r.t |this| and |rect|. Specifically, if |rect| is empty,
|this| will be extended to include it. This might be the behaviour you want, but
I think it's worth calling it out explicitly one way or the other here.

Also, since this method is complementary to IntersectionWith(), maybe move it to
immediately after that method in both header and implementation.

[Hzj_jie, 2017/05/02 23:28:28]

A good point, I failed to notice it. I believe a correct behavior is to ignore
|rect| if it's empty. In DirectX capturer, the |rect| won't be empty. In
desktop_configuration.mm, I do not believe believe pixel_bounds may be empty,
otherwise we would have bug for a long time.

+  void UnionWith(const DesktopRect& rect);
+
  private:
   DesktopRect(int32_t left, int32_t top, int32_t right, int32_t bottom)
       : left_(left), top_(top), right_(right), bottom_(bottom) {