store SkAlphaType inside SkBitmap, on road to support unpremul

tests/BitmapHasherTest.cpp

Index: tests/BitmapHasherTest.cpp
diff --git a/tests/BitmapHasherTest.cpp b/tests/BitmapHasherTest.cpp
index 924cea2dcc0532e81c6848de38f80413825f2916..d6c9e6ab2fbce13f99c928f2184a76785e34a976 100644
--- a/tests/BitmapHasherTest.cpp
+++ b/tests/BitmapHasherTest.cpp
@@ -31,7 +31,7 @@ namespace skiatest {
                               SkColor color) {
             bitmap.setConfig(config, width, height);
             REPORTER_ASSERT(fReporter, bitmap.allocPixels());
-            bitmap.setIsOpaque(true);
+            bitmap.setAlphaType(kOpaque_SkAlphaType);
             bitmap.eraseColor(color);
         }