Fix edge case bugs of canvas arc.

The canvas spec [1] defines how to draw the circumference of arc with various
conditions of startAngle, engAngle and anticlockwise. Current implementation
does not satisfy the spec.

[1] http://www.w3.org/TR/2dcontext2/

TEST=fast/canvas/canvas-arc-circumference.html
BUG=130260

Review URL: https://chromiumcodereview.appspot.com/18286007

git-svn-id: svn://svn.chromium.org/blink/trunk@156647 bbb929c8-8fbe-4397-9dbb-9b2b20218538