Promotes external stylesheets and style elements with the attribute polymer-scope='global' into global scope. This is particularly useful for defining @keyframe rules which currently do not function in scoped or shadow style elements. (See wkb.ug/72462)
// TODO(sorvell): remove when wkb.ug/72462 is addressed.
void installGlobalStyles() {
var style = styleForScope(_STYLE_GLOBAL_SCOPE);
Polymer.applyStyleToScope(style, document.head);
}