replace SkBitmap::Config with SkColorType in gms

add helper installMaskPixels() to SkBitmap

BUG=skia:
R=halcanary@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/169913003

git-svn-id: http://skia.googlecode.com/svn/trunk@13482 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/bitmapshader.cpp b/gm/bitmapshader.cpp
index d1915c9..4649b7e 100644
--- a/gm/bitmapshader.cpp
+++ b/gm/bitmapshader.cpp
@@ -28,7 +28,7 @@
     SkPaint circlePaint;
     circlePaint.setColor(SK_ColorBLACK);
 
-    bm->allocConfigPixels(SkBitmap::kA8_Config, 20, 20);
+    bm->allocPixels(SkImageInfo::MakeA8(20, 20));
     bm->eraseColor(SK_ColorTRANSPARENT);
 
     SkCanvas canvas(*bm);