Implement basic parts of hit regions on canvas2d.

Specification:
http://www.w3.org/TR/2dcontext/#hit-regions

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

BUG=328961

Review URL: https://codereview.chromium.org/300223009

git-svn-id: svn://svn.chromium.org/blink/trunk@176674 bbb929c8-8fbe-4397-9dbb-9b2b20218538