[WebRTC] Add ScreenCapturerDifferWrapper to share Differ across ScreenCapturers

webrtc/modules/desktop_capture/desktop_geometry.cc

Index: webrtc/modules/desktop_capture/desktop_geometry.cc
diff --git a/webrtc/modules/desktop_capture/desktop_geometry.cc b/webrtc/modules/desktop_capture/desktop_geometry.cc
index 1ff7c683c79828a0d8c4e5f9c4c4cb23fd365243..2af8cf826e4dc70193e58569b0e6824eddc140de 100644
--- a/webrtc/modules/desktop_capture/desktop_geometry.cc
+++ b/webrtc/modules/desktop_capture/desktop_geometry.cc
@@ -44,5 +44,15 @@ void DesktopRect::Translate(int32_t dx, int32_t dy) {
   bottom_ += dy;
 }
 
+void DesktopRect::Extend(int32_t left_offset,
+                         int32_t top_offset,
+                         int32_t right_offset,
+                         int32_t bottom_offset) {
+  left_ -= left_offset;
+  top_ -= top_offset;
+  right_ += right_offset;
+  bottom_ += bottom_offset;
+}
+
 }  // namespace webrtc