Add contextmenu attribute for HTMLElement.

contextmenu attribute which enables web pages to specify custom
context menu along with or in place of default platform context
menu for a particular element. The value of the contextmenu
attribute must be the ID of menu element with type popup.

Specifications:
http://www.whatwg.org/specs/web-apps/current-work/#htmlelement
http://www.whatwg.org/specs/web-apps/current-work/#context-menus

BUG=87553

Review URL: https://codereview.chromium.org/464903002

git-svn-id: svn://svn.chromium.org/blink/trunk@180551 bbb929c8-8fbe-4397-9dbb-9b2b20218538