Add support for new canvas ellipse method.

Canvas v5 API adds a new path segment type: ellipse.
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#dom-context-2d-ellipse

Add ellipse API into Path.h because Canvas v5 adds following API also.
   path.ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle, anticlockwise)

When we support Path primitives, we can reuse Path::ellipse.

BUG=130260
TEST=fast/canvas/canvas-ellipse-360-winding.html,
     fast/canvas/canvas-ellipse-circumference.html,
     fast/canvas/canvas-ellipse-connecting-line.html,
     fast/canvas/canvas-ellipse-zero-lineto.html,
     fast/canvas/canvas-ellipse.html

Review URL: https://chromiumcodereview.appspot.com/14298022

git-svn-id: svn://svn.chromium.org/blink/trunk@156828 bbb929c8-8fbe-4397-9dbb-9b2b20218538