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

gm/bigmatrix.cpp

Index: gm/bigmatrix.cpp
diff --git a/gm/bigmatrix.cpp b/gm/bigmatrix.cpp
index 9b49a64ea24407b0748e64781dc0b8efed13c206..b663b2b1ef2192cab2767846d0267b3e7fae18e8 100644
--- a/gm/bigmatrix.cpp
+++ b/gm/bigmatrix.cpp
@@ -59,15 +59,12 @@ protected:
         canvas->drawRect(rect, paint);
 
         SkBitmap bmp;
-        bmp.setConfig(SkBitmap::kARGB_8888_Config, 2, 2);
-        bmp.allocPixels();
-        bmp.lockPixels();
+        bmp.allocN32Pixels(2, 2);
         uint32_t* pixels = reinterpret_cast<uint32_t*>(bmp.getPixels());
         pixels[0] = SkPackARGB32(0xFF, 0xFF, 0x00, 0x00);
         pixels[1] = SkPackARGB32(0xFF, 0x00, 0xFF, 0x00);
         pixels[2] = SkPackARGB32(0x80, 0x00, 0x00, 0x00);
         pixels[3] = SkPackARGB32(0xFF, 0x00, 0x00, 0xFF);
-        bmp.unlockPixels();
         pt.set(30 * SK_Scalar1, 30 * SK_Scalar1);
         m.mapPoints(&pt, 1);
         SkShader* shader = SkShader::CreateBitmapShader(