Implement document.currentScript

http://www.whatwg.org/specs/web-apps/current-work/multipage/dom.html#dom-document-currentscript

document.currentScript reflects the script that is currently being executed.

BUG=240876

Review URL: https://chromiumcodereview.appspot.com/14852011

git-svn-id: svn://svn.chromium.org/blink/trunk@152230 bbb929c8-8fbe-4397-9dbb-9b2b20218538
12 files changed