Partial implementation of CSSVariablesMap for CSS Variables CSSOM

This is a partial implementation of the CSSVariablesMap specification.
This provides Javascript access to CSS Custom Properties (variables).

This implementation is missing the forEach() method which is to come in
a later patch.

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

BUG=231791

Review URL: https://chromiumcodereview.appspot.com/18311002

git-svn-id: svn://svn.chromium.org/blink/trunk@154472 bbb929c8-8fbe-4397-9dbb-9b2b20218538