Factor code to rotate a canvas about a point.

bench/BitmapBench.cpp

Index: bench/BitmapBench.cpp
diff --git a/bench/BitmapBench.cpp b/bench/BitmapBench.cpp
index 812f8c32366ab6e0b60f24f69e73f4c06f3f2dc1..e68b6515b530067e096312a317a2a308b3341641 100644
--- a/bench/BitmapBench.cpp
+++ b/bench/BitmapBench.cpp
@@ -239,10 +239,7 @@ protected:
         if (fFlags & kRotate_Flag) {
             const SkScalar x = SkIntToScalar(dim.fWidth) / 2;
             const SkScalar y = SkIntToScalar(dim.fHeight) / 2;
-
-            canvas->translate(x, y);
-            canvas->rotate(SkIntToScalar(35));
-            canvas->translate(-x, -y);
+            sk_tool_utils::rotate_about(canvas, SkIntToScalar(35), x, y);
         }
         INHERITED::onDraw(loops, canvas);
     }