Re-enable SSE4.

I will roll this into Chrome with https://codereview.chromium.org/332393003.

BUG=skia:
R=reed@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/357593003
diff --git a/src/opts/SkBlitRow_opts_SSE4.h b/src/opts/SkBlitRow_opts_SSE4.h
index b6ab140e..6c6276e 100644
--- a/src/opts/SkBlitRow_opts_SSE4.h
+++ b/src/opts/SkBlitRow_opts_SSE4.h
@@ -18,9 +18,7 @@
                                                const SkPMColor* SK_RESTRICT src,
                                                int count, U8CPU alpha);
 
-// Temporarily disabled.  Chrome canary bot fails to link chrome with error:
-//   lib/libskia.so: error: undefined reference to 'S32A_Opaque_BlitRow32_SSE4_asm'
-//#define SK_ATT_ASM_SUPPORTED
+#define SK_ATT_ASM_SUPPORTED
 #endif
 
 #endif