Factor code to rotate a canvas about a point.

gm/dftext.cpp

Index: gm/dftext.cpp
diff --git a/gm/dftext.cpp b/gm/dftext.cpp
index 42d650ed2f4508189089336ee1a1e2b8878ef3ea..e64c1df4ca805282889a476f22acf534bb774cfd 100644
--- a/gm/dftext.cpp
+++ b/gm/dftext.cpp
@@ -10,6 +10,7 @@
 #include "SkStream.h"
 #include "SkSurface.h"
 #include "SkTypeface.h"
+#include "sk_tool_utils.h"
 
 class DFTextGM : public skiagm::GM {
 public:
@@ -33,14 +34,6 @@ protected:
         return SkISize::Make(1024, 768);
     }
 
-    static void rotate_about(SkCanvas* canvas,
-        SkScalar degrees,
-        SkScalar px, SkScalar py) {
-        canvas->translate(px, py);
-        canvas->rotate(degrees);
-        canvas->translate(-px, -py);
-    }
-
     virtual void onDraw(SkCanvas* inputCanvas) override {
         SkScalar textSizes[] = { 9.0f, 9.0f*2.0f, 9.0f*5.0f, 9.0f*2.0f*5.0f };
         SkScalar scales[] = { 2.0f*5.0f, 5.0f, 2.0f, 1.0f };
@@ -95,7 +88,7 @@ protected:
 
             SkAutoCanvasRestore acr(canvas, true);
             canvas->translate(SkIntToScalar(10 + i * 200), -80);
-            rotate_about(canvas, SkIntToScalar(i * 5), rotX, rotY);
+            sk_tool_utils::rotate_about(canvas, SkIntToScalar(i * 5), rotX, rotY);
             for (int ps = 6; ps <= 32; ps += 3) {
                 paint.setTextSize(SkIntToScalar(ps));
                 canvas->drawText(text, textLen, rotX, rotY, paint);