store SkAlphaType inside SkBitmap, on road to support unpremul

BUG=
R=bsalomon@google.com, scroggo@google.com

Review URL: https://codereview.chromium.org/25275004

git-svn-id: http://skia.googlecode.com/svn/trunk@11877 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/samplecode/SampleTinyBitmap.cpp b/samplecode/SampleTinyBitmap.cpp
index 42866d0..dc7157f 100644
--- a/samplecode/SampleTinyBitmap.cpp
+++ b/samplecode/SampleTinyBitmap.cpp
@@ -57,9 +57,8 @@
 
     static void setBitmapOpaque(SkBitmap* bm, bool isOpaque) {
         SkAutoLockPixels alp(*bm);  // needed for ctable
-        bm->setIsOpaque(isOpaque);
+        bm->setAlphaType(isOpaque ? kOpaque_SkAlphaType : kPremul_SkAlphaType);
 #if 0
-        // TODO - I think we just want to not allow this anymore
         SkColorTable* ctable = bm->getColorTable();
         if (ctable) {
             ctable->setIsOpaque(isOpaque);