Do not scale when drawing ellipse

There is no need do scaling when drawing ellipse and it would also hurt
precision. This CL enhances original Path::addArc() a bit to avoid
scaling. A new layout test to fill ellipse is also added.

BUG=293220

Review URL: https://codereview.chromium.org/23513060

git-svn-id: svn://svn.chromium.org/blink/trunk@159322 bbb929c8-8fbe-4397-9dbb-9b2b20218538