partial switch over to sp usage of shaders

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1776973003

Review URL: https://codereview.chromium.org/1776973003
diff --git a/gm/bigmatrix.cpp b/gm/bigmatrix.cpp
index 2d2b7d9..b39f2ea 100644
--- a/gm/bigmatrix.cpp
+++ b/gm/bigmatrix.cpp
@@ -55,12 +55,8 @@
         SkMatrix s;
         s.reset();
         s.setScale(SK_Scalar1 / 1000, SK_Scalar1 / 1000);
-        SkShader* shader = SkShader::CreateBitmapShader(
-                                            bmp,
-                                            SkShader::kRepeat_TileMode,
-                                            SkShader::kRepeat_TileMode,
-                                            &s);
-        paint.setShader(shader)->unref();
+        paint.setShader(SkShader::MakeBitmapShader(bmp, SkShader::kRepeat_TileMode,
+                                                   SkShader::kRepeat_TileMode, &s));
         paint.setAntiAlias(false);
         paint.setFilterQuality(kLow_SkFilterQuality);
         rect.setLTRB(pt.fX - small, pt.fY - small,