hide SkBitmap::setConfig

patch from issue 325733002

TBR=scroggo

Author: reed@chromium.org

Review URL: https://codereview.chromium.org/322963002
diff --git a/tools/skdiff_utils.cpp b/tools/skdiff_utils.cpp
index 5ca3d80..5c9aae2 100644
--- a/tools/skdiff_utils.cpp
+++ b/tools/skdiff_utils.cpp
@@ -144,11 +144,9 @@
     if (w != drp->fComparison.fBitmap.width() || h != drp->fComparison.fBitmap.height()) {
         drp->fResult = DiffRecord::kDifferentSizes_Result;
     } else {
-        drp->fDifference.fBitmap.setConfig(SkBitmap::kARGB_8888_Config, w, h);
-        drp->fDifference.fBitmap.allocPixels();
+        drp->fDifference.fBitmap.allocN32Pixels(w, h);
 
-        drp->fWhite.fBitmap.setConfig(SkBitmap::kARGB_8888_Config, w, h);
-        drp->fWhite.fBitmap.allocPixels();
+        drp->fWhite.fBitmap.allocN32Pixels(w, h);
 
         SkASSERT(DiffRecord::kUnknown_Result == drp->fResult);
         compute_diff(drp, dmp, colorThreshold);