Annotate Polymer function with @global.

[aomarks]

This will let Analyzer discover it thanks to
Polymer/polymer-analyzer#774.

Part of https://github.com/Polymer/gen-typescript-declarations/issues/23

[TimvdLippe]

I am okay with an extra annotation, but it leaves me wondering why detecting an assignment starting with window. wouldn't work. Is that because the polymerFn is used instead of Polymer?

[aomarks]

Our approach in analyzer has so far generally been to just follow the @memberof annotations, rather than trying to understand the LHS of the assignment expression. If a function doesn't have @memberof, it would be totally ignored. So I just extended this to also respect @global, since @memberof window was a bit weird (see Polymer/polymer-analyzer#774). I guess we could have had special handling for window instead, this was just simpler to do. Note it is a JSDoc thing: http://usejsdoc.org/tags-global.html.