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

gm/clippedbitmapshaders.cpp

Index: gm/clippedbitmapshaders.cpp
diff --git a/gm/clippedbitmapshaders.cpp b/gm/clippedbitmapshaders.cpp
index 4087c89c38b3b189fc4333d3586965a2ebf3e3c7..72c04742608c1591e2a02f0dd62c4fd0cb10b36f 100644
--- a/gm/clippedbitmapshaders.cpp
+++ b/gm/clippedbitmapshaders.cpp
@@ -27,15 +27,12 @@ namespace skiagm {
 
 static SkBitmap create_bitmap() {
     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] = SkPreMultiplyColor(SK_ColorRED);
     pixels[1] = SkPreMultiplyColor(SK_ColorGREEN);
     pixels[2] = SkPreMultiplyColor(SK_ColorBLACK);
     pixels[3] = SkPreMultiplyColor(SK_ColorBLUE);
-    bmp.unlockPixels();
 
     return bmp;
 }