Factor code to rotate a canvas about a point.

include/core/SkCanvas.h

Index: include/core/SkCanvas.h
diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index 26458696bc4d81a4b1a9b304aa847616e93c011a..97aa3c7d8e343e06a1464615fb6edc788afc8148 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -419,11 +419,18 @@ public:
     */
     void scale(SkScalar sx, SkScalar sy);
 
-    /** Preconcat the current matrix with the specified rotation.
+    /** Preconcat the current matrix with the specified rotation about the origin.
         @param degrees  The amount to rotate, in degrees
     */
     void rotate(SkScalar degrees);
 
+    /** Preconcat the current matrix with the specified rotation about a given point.
+        @param degrees  The amount to rotate, in degrees
+        @param px  The x coordinate of the point to rotate about.
+        @param py  The y coordinate of the point to rotate about.
+    */
+    void rotate(SkScalar degrees, SkScalar px, SkScalar py);
+
     /** Preconcat the current matrix with the specified skew.
         @param sx   The amount to skew in X
         @param sy   The amount to skew in Y