[WebRTC] Add ScreenCapturerDifferWrapper to share Differ across ScreenCapturers

webrtc/modules/desktop_capture/differ_block.h

Index: webrtc/modules/desktop_capture/differ_block.h
diff --git a/webrtc/modules/desktop_capture/differ_block.h b/webrtc/modules/desktop_capture/differ_block.h
index e1d487d68bac9da48dd59b97e698237c8c1bb5ed..97fe3bc544f7dff37de97d97036a6ec2b5be91aa 100644
--- a/webrtc/modules/desktop_capture/differ_block.h
+++ b/webrtc/modules/desktop_capture/differ_block.h
@@ -22,6 +22,17 @@ const int kBlockSize = 32;
 // Format: BGRA 32 bit.
 const int kBytesPerPixel = 4;
 
+// Low level function to compare 2 vectors of pixels of size kBlockSize. Returns
+// whether the blocks differ.
+bool VectorDifference(const uint8_t* image1, const uint8_t* image2);
+
+// Low level function to compare 2 blocks of pixels of size
+// (kBlockSize, |height|).  Returns whether the blocks differ.
+bool BlockDifference(const uint8_t* image1,
+                     const uint8_t* image2,
+                     int height,
+                     int stride);
+
 // Low level function to compare 2 blocks of pixels of size
 // (kBlockSize, kBlockSize).  Returns whether the blocks differ.
 bool BlockDifference(const uint8_t* image1,