Implement canvas2d direction attribute

The attribute, 'direction', represents the text directionality.
This is an important attribute for the bi-directional text within the CanvasRenderingContext2D.
The allowed set of values are 'rtl', 'ltr' and 'inherit'.

Specification URL: http://www.whatwg.org/specs/web-apps/current-work/multipage/scripting.html#dom-context-2d-direction

Intent to implement: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/KWKkTpKiXXM
BUG=277209

Review URL: https://codereview.chromium.org/468483003

git-svn-id: svn://svn.chromium.org/blink/trunk@180984 bbb929c8-8fbe-4397-9dbb-9b2b20218538