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

gm/samplerstress.cpp

Index: gm/samplerstress.cpp
diff --git a/gm/samplerstress.cpp b/gm/samplerstress.cpp
index 93a26dd49c9691e9546a0ad1b69a37dd83aa7cd3..2ddac3a1ae2e2e2424c777f06859e0e0dae2d3a9 100644
--- a/gm/samplerstress.cpp
+++ b/gm/samplerstress.cpp
@@ -47,13 +47,7 @@ protected:
         static const int xSize = 16;
         static const int ySize = 16;
 
-        fTexture.setConfig(SkBitmap::kARGB_8888_Config,
-                           xSize,
-                           ySize,
-                           xSize*sizeof(SkColor));
-
-        fTexture.allocPixels();
-        fTexture.lockPixels();
+        fTexture.allocN32Pixels(xSize, ySize);
         SkPMColor* addr = fTexture.getAddr32(0, 0);
 
         for (int y = 0; y < ySize; ++y) {
@@ -69,8 +63,6 @@ protected:
             }
         }
 
-        fTexture.unlockPixels();
-
         fTextureCreated = true;
     }