Factor code to rotate a canvas about a point.

samplecode/SampleHT.cpp

Index: samplecode/SampleHT.cpp
diff --git a/samplecode/SampleHT.cpp b/samplecode/SampleHT.cpp
index d0cd3a54996dce2211c17b9a4637b75f29a5ff92..273ca1ae9bf25fab56a9a1b08e3b87bd6f8d5640 100644
--- a/samplecode/SampleHT.cpp
+++ b/samplecode/SampleHT.cpp
@@ -13,6 +13,7 @@
 #include "SkInterpolator.h"
 #include "SkPictureRecorder.h"
 #include "SkRandom.h"
+#include "sk_tool_utils.h"
 
 const SkRect gUnitSquare = { -1, -1, 1, 1 };
 
@@ -95,9 +96,7 @@ public:
             fColor = floats_to_color(values);
 
             canvas->save();
-            canvas->translate(fR.centerX(), fR.centerY());
-            canvas->rotate(values[4]);
-            canvas->translate(-fR.centerX(), -fR.centerY());
+            sk_tool_utils::rotate_about(canvas, values[4], fR.centerX(), fR.centerY());
 
             switch (res) {
                 case SkInterpolator::kFreezeEnd_Result: