Implement addPath() method for Path object

Intent to implement on blink-dev:
https://groups.google.com/a/chromium.org/forum/#!searchin/blink-dev/addPath/blink-dev/YY-FmD8Efhc/y4Ag3pGF-toJ

BUG=159839

Review URL: https://codereview.chromium.org/170503002

git-svn-id: svn://svn.chromium.org/blink/trunk@168561 bbb929c8-8fbe-4397-9dbb-9b2b20218538