Refactor BlockDifference() to use MediaInitializer().

media/video/capture/screen/differ_block.h

Index: media/video/capture/screen/differ_block.h
diff --git a/media/video/capture/screen/differ_block.h b/media/video/capture/screen/differ_block.h
index 8b211e2e65d2d887fb22ee51e6b9fca022375213..d3c5d33b362e5696f356ba28a1afa3355b10f9fa 100644
--- a/media/video/capture/screen/differ_block.h
+++ b/media/video/capture/screen/differ_block.h
@@ -10,12 +10,19 @@
 
 namespace media {
 
-// Size (in pixels) of each square block used for diffing. This must be a
-// multiple of sizeof(uint64)/8.
-const int kBlockSize = 32;
-
-// Format: BGRA 32 bit.
-const int kBytesPerPixel = 4;
+enum {
+  // Size (in pixels) of each square block used for diffing. This must be a
+  // multiple of sizeof(uint64)/8.
+  kBlockSize = 32,
+
+  // Format: BGRA 32 bit.
+  kBytesPerPixel = 4,
+};
+
+// Selects the optimal CPU specific BlockDifference() method at runtime.  Must
+// be called prior to using BlockDifference.  Called during media library init;
+// most users should not need to call this function.
+MEDIA_EXPORT void InitializeCPUSpecificBlockDifference();
 
 // Low level functions to compare 2 blocks of pixels. Zero means the blocks
 // are identical. One - the blocks are different.