add legacy/helper allocN32Pixels, and convert gm to use it

include/core/SkBitmap.h

Index: include/core/SkBitmap.h
diff --git a/include/core/SkBitmap.h b/include/core/SkBitmap.h
index af0c8b87e7a643fbf7fbf7b193fcb53b8ffcf010..8059e262f3d4c7d13abf864f52826ae80578fc28 100644
--- a/include/core/SkBitmap.h
+++ b/include/core/SkBitmap.h
@@ -267,7 +267,21 @@ public:
     bool allocPixels(const SkImageInfo& info) {
         return this->allocPixels(info, NULL, NULL);
     }
-    
+
+    /**
+     *  Legacy helper function, which creates an SkImageInfo from the specified
+     *  config and then calls allocPixels(info).
+     */
+    bool allocConfigPixels(Config, int width, int height, bool isOpaque = false);
+
+    bool allocN32Pixels(int width, int height, bool isOpaque = false) {
+        SkImageInfo info = SkImageInfo::MakeN32Premul(width, height);
+        if (isOpaque) {
+            info.fAlphaType = kOpaque_SkAlphaType;
+        }
+        return this->allocPixels(info);
+    }
+
     /**
      *  Install a pixelref that wraps the specified pixels and rowBytes, and
      *  optional ReleaseProc and context. When the pixels are no longer