Add forEach() to CSSVariablesMap

This adds an implementation of forEach from WebIDL MapClass to
CSSVariablesMap.

Specification:
http://dev.w3.org/2006/webapi/WebIDL/#MapClass
http://dev.w3.org/csswg/css-variables/#the-CSSVariablesMap-interface

BUG=231791

Review URL: https://chromiumcodereview.appspot.com/21006006

git-svn-id: svn://svn.chromium.org/blink/trunk@158524 bbb929c8-8fbe-4397-9dbb-9b2b20218538