add new copyTo version to SkBitmap, which takes SkColorType

BUG=skia:
R=scroggo@google.com, halcanary@google.com, bsalomon@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/171723007

git-svn-id: http://skia.googlecode.com/svn/trunk@13553 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/samplecode/SampleDither.cpp b/samplecode/SampleDither.cpp
index 57c7613..c0ce299 100644
--- a/samplecode/SampleDither.cpp
+++ b/samplecode/SampleDither.cpp
@@ -118,7 +118,7 @@
         make_bm(&fBM);
         make_bm(&fBMPreDither);
         pre_dither(fBMPreDither);
-        fBM.copyTo(&fBM16, SkBitmap::kARGB_4444_Config);
+        fBM.copyTo(&fBM16, kARGB_4444_SkColorType);
 
         fAngle = 0;