add optional SkAlphaType parameter to notifyPixelsChanged

src/core/SkBitmap.cpp

Index: src/core/SkBitmap.cpp
diff --git a/src/core/SkBitmap.cpp b/src/core/SkBitmap.cpp
index 707438636dc279cedcd54263988cc186b95140fd..825b1dcecad7587a8d0f2520c296f49f254849c9 100644
--- a/src/core/SkBitmap.cpp
+++ b/src/core/SkBitmap.cpp
@@ -321,7 +321,12 @@ bool SkBitmap::setAlphaType(SkAlphaType alphaType) {
     if (!validate_alphaType(this->config(), alphaType, &alphaType)) {
         return false;
     }
-    fAlphaType = SkToU8(alphaType);
+    if (fAlphaType != alphaType) {
+        fAlphaType = SkToU8(alphaType);
+        if (fPixelRef) {
+            fPixelRef->notifyPixelsChanged(alphaType);
+        }
+    }
     return true;
 }