store SkAlphaType inside SkBitmap, on road to support unpremul

src/core/SkBitmapDevice.cpp

Index: src/core/SkBitmapDevice.cpp
diff --git a/src/core/SkBitmapDevice.cpp b/src/core/SkBitmapDevice.cpp
index eee201881a879d340c3ed594ba4b9b55340ecab7..ff68481538286fd25f92734f527a3134de03b1f4 100644
--- a/src/core/SkBitmapDevice.cpp
+++ b/src/core/SkBitmapDevice.cpp
@@ -27,9 +27,9 @@ SkBitmapDevice::SkBitmapDevice(const SkBitmap& bitmap, const SkDeviceProperties&
 }
 
 SkBitmapDevice::SkBitmapDevice(SkBitmap::Config config, int width, int height, bool isOpaque) {
-    fBitmap.setConfig(config, width, height);
+    fBitmap.setConfig(config, width, height, 0, isOpaque ?
+                      kOpaque_SkAlphaType : kPremul_SkAlphaType);
     fBitmap.allocPixels();
-    fBitmap.setIsOpaque(isOpaque);
     if (!isOpaque) {
         fBitmap.eraseColor(SK_ColorTRANSPARENT);
     }
@@ -39,9 +39,9 @@ SkBitmapDevice::SkBitmapDevice(SkBitmap::Config config, int width, int height, b
                                const SkDeviceProperties& deviceProperties)
     : SkBaseDevice(deviceProperties) {
 
-    fBitmap.setConfig(config, width, height);
+    fBitmap.setConfig(config, width, height, 0, isOpaque ?
+                      kOpaque_SkAlphaType : kPremul_SkAlphaType);
     fBitmap.allocPixels();
-    fBitmap.setIsOpaque(isOpaque);
     if (!isOpaque) {
         fBitmap.eraseColor(SK_ColorTRANSPARENT);
     }