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

This is an intermediate api, but might help us quickly get to a point where
no one is creating bitmaps in a 2-step process (setConfig + alloc).

BUG=skia:
R=halcanary@google.com

Review URL: https://codereview.chromium.org/140593005

git-svn-id: http://skia.googlecode.com/svn/trunk@13182 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/downsamplebitmap.cpp b/gm/downsamplebitmap.cpp
index 034538c..4b97804 100644
--- a/gm/downsamplebitmap.cpp
+++ b/gm/downsamplebitmap.cpp
@@ -99,8 +99,7 @@
       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 @@
       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 @@
                   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();