[WebRTC] Add ScreenCapturerDifferWrapper to share Differ across ScreenCapturers

webrtc/modules/desktop_capture/desktop_geometry.cc

 /*
  *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 26 matching lines @@
   }
 }
 
 void DesktopRect::Translate(int32_t dx, int32_t dy) {
   left_ += dx;
   top_ += dy;
   right_ += dx;
   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