use colortype instead of config

src/core/SkBitmap_scroll.cpp

Index: src/core/SkBitmap_scroll.cpp
diff --git a/src/core/SkBitmap_scroll.cpp b/src/core/SkBitmap_scroll.cpp
index e9c886f4a0e7c43d5a1762fdf6f514660adf87e1..00a72aa1cab983f5a9ad089bbca7298c2da2c445 100644
--- a/src/core/SkBitmap_scroll.cpp
+++ b/src/core/SkBitmap_scroll.cpp
@@ -11,7 +11,7 @@
 bool SkBitmap::scrollRect(const SkIRect* subset, int dx, int dy,
                           SkRegion* inval) const
 {
-    if (this->isImmutable()) {
+    if (this->isImmutable() || kUnknown_SkColorType == this->colorType()) {
         return false;
     }
 
@@ -23,25 +23,7 @@ bool SkBitmap::scrollRect(const SkIRect* subset, int dx, int dy,
                 tmp.scrollRect(NULL, dx, dy, inval);
     }
 
-    int shift;
-
-    switch (this->config()) {
-    case kIndex8_Config:
-    case kA8_Config:
-        shift = 0;
-        break;
-    case kARGB_4444_Config:
-    case kRGB_565_Config:
-        shift = 1;
-        break;
-    case kARGB_8888_Config:
-        shift = 2;
-        break;
-    default:
-        // can't scroll this config
-        return false;
-    }
-
+    int shift = this->bytesPerPixel() >> 1;
     int width = this->width();
     int height = this->height();