PSA: Polymer externs have moved

[Dan Beam]

If you don't write Polymer in Chrom{e,ium}: feel free to stop reading now.

Polymer's externs have moved from Polymer labs' github:

https://github.com/PolymerLabs/polymer-externs

To closure compiler:

https://github.com/google/closure-compiler/blob/master/contrib/externs/polymer-1.0.js

As of this CL, we're now using the new (Closure Compiler) location:

https://codereview.chromium.org/2052973002/

Note: Updating Polymer (i.e. third_party/polymer/v1_0/reproduce.sh) will NOT update polymer externs as it previously has.  Updating these externs are now part of updating Closure Compiler (i.e. third_party/closure_compiler/roll_closure_compiler).

Note: internal (Google) CLs are now the preferred way to change these externs.  Github pull requests will still work, but may take the author/approver more time/effort.

-- Dan