Add path option(path2d) for hit regions on canvas2d.

The specification:
  http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#hitregionoptions

Intent to implement:
  https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/tRFguWa-oZc

BUG=328961

Review URL: https://codereview.chromium.org/354333002

git-svn-id: svn://svn.chromium.org/blink/trunk@177559 bbb929c8-8fbe-4397-9dbb-9b2b20218538