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/bench/RepeatTileBench.cpp b/bench/RepeatTileBench.cpp
index 612a149..957ec2a 100644
--- a/bench/RepeatTileBench.cpp
+++ b/bench/RepeatTileBench.cpp
@@ -115,7 +115,8 @@
     virtual void onPreDraw() SK_OVERRIDE {
         fBitmap.allocPixels();
         fBitmap.eraseColor(fIsOpaque ? SK_ColorWHITE : 0);
-        fBitmap.setIsOpaque(fIsOpaque);
+        fBitmap.setAlphaType(fIsOpaque ?
+                             kOpaque_SkAlphaType : kPremul_SkAlphaType);
 
         draw_into_bitmap(fBitmap);