Factor code to rotate a canvas about a point.

samplecode/SampleDither.cpp

Index: samplecode/SampleDither.cpp
diff --git a/samplecode/SampleDither.cpp b/samplecode/SampleDither.cpp
index 99b33f0f0ef6dfafe18106e255e157b733267a7a..9e127576a89dfe7e132a1120f10d09350b14cd51 100644
--- a/samplecode/SampleDither.cpp
+++ b/samplecode/SampleDither.cpp
@@ -19,6 +19,7 @@
 #include "SkColorPriv.h"
 #include "SkColorFilter.h"
 #include "SkDither.h"
+#include "sk_tool_utils.h"
 
 static void draw_sweep(SkCanvas* c, int width, int height, SkScalar angle) {
     SkRect  r;
@@ -37,10 +38,7 @@ static void draw_sweep(SkCanvas* c, int width, int height, SkScalar angle) {
                                             colors, nullptr, SK_ARRAY_COUNT(colors)));
 
     SkAutoCanvasRestore acr(c, true);
-
-    c->translate(r.centerX(), r.centerY());
-    c->rotate(angle);
-    c->translate(-r.centerX(), -r.centerY());
+    sk_tool_utils::rotate_about(c, angle, r.centerX(), r.centerY());
 
     SkRect bounds = r;
     r.inset(p.getStrokeWidth(), p.getStrokeWidth());