canvas: arc can never cover an angle greater than 2pi radians.

Recently, the Canvas spec was changed as title.
Currently, arc in Blink can cover greater than 2pi radians. So Blink needs to
follow the spec change.

W3 discussion: https://www.w3.org/Bugs/Public/show_bug.cgi?id=23050

New spec about arc :
http://www.whatwg.org/specs/web-apps/current-work/#dom-context-2d-arc
If the anticlockwise argument is false and endAngle-startAngle is equal to or
greater than 2pi, or, if the anticlockwise argument is true and
startAngle-endAngle is equal to or greater than 2pi, then the arc is the whole
circumference of this ellipse, and the point at startAngle along this circle's
circumference, measured in radians clockwise from the ellipse's semi-major axis,
acts as both the start point and the end point.

Otherwise, the points at startAngle and endAngle along this circle's
circumference, measured in radians clockwise from the ellipse's semi-major axis,
are the start and end points respectively, and the arc is the path along the
circumference of this ellipse from the start point to the end point, going
anti-clockwise if the anticlockwise argument is true, and clockwise otherwise.
Since the points are on the ellipse, as opposed to being simply angles from
zero, the arc can never cover an angle greater than 2pi radians.

Even if the arc covers the entire circumference of the ellipse and there are no
other points in the subpath, the path is not closed unless the closePath()
method is appropriately invoked.

TEST=following tests cover this change.
fast/canvas/canvas-arc-circumference-fill.html,
fast/canvas/canvas-arc-circumference.html,
fast/canvas/canvas-ellipse-circumference-fill.html,
fast/canvas/canvas-ellipse-circumference.html,
fast/canvas/canvas-ellipse-zero-lineto.html
BUG=324045

Review URL: https://codereview.chromium.org/96063007

git-svn-id: svn://svn.chromium.org/blink/trunk@163797 bbb929c8-8fbe-4397-9dbb-9b2b20218538