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

gm/mixedxfermodes.cpp

Index: gm/mixedxfermodes.cpp
diff --git a/gm/mixedxfermodes.cpp b/gm/mixedxfermodes.cpp
index e3ce91a9dcb0b210a006651449b0f15a8bfca3fc..45623255571e26984ebb286a9124833cbc3b1f38 100644
--- a/gm/mixedxfermodes.cpp
+++ b/gm/mixedxfermodes.cpp
@@ -84,11 +84,8 @@ protected:
                                                     0xFFCCCCCC,
                                                     0xFFFFFFFF };
             SkBitmap bitmap;
-            bitmap.setConfig(SkBitmap::kARGB_8888_Config, 2, 2, 2 * sizeof(uint32_t));
-            bitmap.allocPixels();
-            bitmap.lockPixels();
+            bitmap.allocN32Pixels(2, 2);
             memcpy(bitmap.getPixels(), kCheckerPixelData, sizeof(kCheckerPixelData));
-            bitmap.unlockPixels();
             fBG.reset(SkShader::CreateBitmapShader(bitmap,
                                                    SkShader::kRepeat_TileMode,
                                                    SkShader::kRepeat_TileMode));