store SkAlphaType inside SkBitmap, on road to support unpremul

gm/bleed.cpp

Index: gm/bleed.cpp
diff --git a/gm/bleed.cpp b/gm/bleed.cpp
index dc7e96fbb122716c76d397de1cf5d59eda399354..0d093940decc73da421833cb8336b96bf267b98e 100644
--- a/gm/bleed.cpp
+++ b/gm/bleed.cpp
@@ -19,7 +19,8 @@
 static void make_red_ringed_bitmap(SkBitmap* result, int width, int height) {
     SkASSERT(0 == width % 2 && 0 == width % 2);
 
-    result->setConfig(SkBitmap::kARGB_8888_Config, width, height);
+    result->setConfig(SkBitmap::kARGB_8888_Config, width, height, 0,
+                      kOpaque_SkAlphaType);
     result->allocPixels();
     SkAutoLockPixels lock(*result);
 
@@ -56,7 +57,6 @@ static void make_red_ringed_bitmap(SkBitmap* result, int width, int height) {
     for (int x = 0; x < width; ++x) {
         scanline[x] = SK_ColorRED;
     }
-    result->setIsOpaque(true);
     result->setImmutable();
 }