Get Polymer externs from closure/github (instead of Polymer/bower)

R=jlklein@chromium.org
TBR=fukino@chromium.org  # for trivial path change
BUG=none

Review-Url: https://codereview.chromium.org/2052973002
Cr-Commit-Position: refs/heads/master@{#398991}
7 files changed