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/TileBench.cpp b/bench/TileBench.cpp
index 60312a3..03a758c 100644
--- a/bench/TileBench.cpp
+++ b/bench/TileBench.cpp
@@ -48,11 +48,10 @@
         , fDoScale(doScale) {
         SkBitmap bm;
 
-        bm.setConfig(SkBitmap::kARGB_8888_Config, kWidth, kHeight);
-
+        bm.setConfig(SkBitmap::kARGB_8888_Config, kWidth, kHeight, 0,
+                     kOpaque_SkAlphaType);
         bm.allocPixels();
         bm.eraseColor(SK_ColorWHITE);
-        bm.setIsOpaque(true);
 
         create_gradient(&bm);