Templatizer: Variables in the parent scope are not passed during initialization

[kevinpschaaf]

When the templatized template is not the templatizer client, variables in the parent scope are not passed during initialization.

Originally filed as: PolymerElements/iron-list#107

Root caused to https://github.com/Polymer/polymer/blob/master/src/lib/template/templatizer.html#L385-L389; parent props need to be pulled from the rather than the element.

[kevinpschaaf]

PR: #2508