Support Path.addCircle (flutter/engine#4783)

* add Path.addCircle

* docs formatting

* revert path.cc/path.h; remove addCircle and update docs on addOval

* add missing file.

engine/src/flutter/AUTHORS

@@ -7,3 +7,4 @@ Google Inc.
 Jim Simon <jim.j.simon@gmail.com>
 Ali Bitek <alibitek@protonmail.ch>
 Jacob Greenfield <jacob.greenfield.256@gmail.com>
+Dan Field <dfield@gmail.com>

engine/src/flutter/lib/ui/painting.dart

@@ -1513,6 +1513,9 @@ class Path extends NativeFieldWrapperClass2 {
 
   /// Adds a new subpath that consists of a curve that forms the
   /// ellipse that fills the given rectangle.
+  /// 
+  /// To add a circle, pass an appropriate rectangle as `oval`. [Rect.fromCircle] 
+  /// can be used to easily describe the circle's center [Offset] and radius.
   void addOval(Rect oval) {
     assert(_rectIsValid(oval));
     _addOval(oval.left, oval.top, oval.right, oval.bottom);