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

gm/downsamplebitmap.cpp

Index: gm/downsamplebitmap.cpp
diff --git a/gm/downsamplebitmap.cpp b/gm/downsamplebitmap.cpp
index 034538c15c08bbe8864f5820a66b3be34d9a7e38..4b97804b01f74e70c627f5ecc80319d37a2eb943 100644
--- a/gm/downsamplebitmap.cpp
+++ b/gm/downsamplebitmap.cpp
@@ -99,8 +99,7 @@ class DownsampleBitmapTextGM: public DownsampleBitmapGM {
       float fTextSize;
 
       virtual void make_bitmap() SK_OVERRIDE {
-          fBM.setConfig(SkBitmap::kARGB_8888_Config, int(fTextSize * 8), int(fTextSize * 6));
-          fBM.allocPixels();
+          fBM.allocN32Pixels(int(fTextSize * 8), int(fTextSize * 6));
           SkCanvas canvas(fBM);
           canvas.drawColor(SK_ColorWHITE);
 
@@ -135,9 +134,7 @@ class DownsampleBitmapCheckerboardGM: public DownsampleBitmapGM {
       int fNumChecks;
 
       virtual void make_bitmap() SK_OVERRIDE {
-          fBM.setConfig(SkBitmap::kARGB_8888_Config, fSize, fSize);
-          fBM.allocPixels();
-          SkAutoLockPixels lock(fBM);
+          fBM.allocN32Pixels(fSize, fSize);
           for (int y = 0; y < fSize; ++y) {
               for (int x = 0; x < fSize; ++x) {
                   SkPMColor* s = fBM.getAddr32(x, y);
@@ -183,8 +180,7 @@ class DownsampleBitmapImageGM: public DownsampleBitmapGM {
                   SkImageDecoder::kDecodePixels_Mode);
               SkDELETE(codec);
           } else {
-              fBM.setConfig(SkBitmap::kARGB_8888_Config, 1, 1);
-              fBM.allocPixels();
+              fBM.allocN32Pixels(1, 1);
               *(fBM.getAddr32(0,0)) = 0xFF0000FF; // red == bad
           }
           fSize = fBM.height();