Support currentTransform in 2D Canvas.

Canvas v5 has a currentTransform that is an SVGMatrix.
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#dom-context-2d-currenttransform

[Immutable] extended attribute is used, because canvas must not change CTM in
the following case.
    var matrix = context.currentTransform;
    matrix.a = 2;

BUG=277107
TEST=fast/canvas/canvas-currentTransform.html

Review URL: https://codereview.chromium.org/24233004

git-svn-id: svn://svn.chromium.org/blink/trunk@159303 bbb929c8-8fbe-4397-9dbb-9b2b20218538