Add support to populate custom context menu items.

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.
This CL enables ContextMenuController to populate custom context
menu items specified by contextmenu attribute on any element.

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

BUG=87553

Review URL: https://codereview.chromium.org/492753002

git-svn-id: svn://svn.chromium.org/blink/trunk@180896 bbb929c8-8fbe-4397-9dbb-9b2b20218538