Add a RuntimeEnabledFeatures flag to implement contextmenu attribute.

Link to "Intent to implement":
http://goo.gl/UtWWY8

BUG=87553
NOTRY=true

Review URL: https://codereview.chromium.org/443773004

git-svn-id: svn://svn.chromium.org/blink/trunk@179576 bbb929c8-8fbe-4397-9dbb-9b2b20218538