Update Polymer to 1.2.4
Update Polymer to 1.2.4

Also pulls in an updated webcomponentjs (0.7.20) and
polymerelements/test-fixture (1.1.0)

Change-Id: I666d58582884b05231b2db8d16b79ab1c9a5bcf8

qtproject/qtqa-gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,MerritCR/merrit,gerrit-review/gerrit,qtproject/qtqa-gerrit,joshuawilson/merrit,GerritCodeReview/gerrit,joshuawilson/merrit,gerrit-review/gerrit,WANdisco/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,gerrit-review/gerrit,MerritCR/merrit,MerritCR/merrit,joshuawilson/merrit,joshuawilson/merrit,gerrit-review/gerrit,qtproject/qtqa-gerrit,gerrit-review/gerrit,gerrit-review/gerrit,MerritCR/merrit,WANdisco/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,gerrit-review/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,joshuawilson/merrit,GerritCodeReview/gerrit,joshuawilson/merrit,MerritCR/merrit,GerritCodeReview/gerrit,joshuawilson/merrit,WANdisco/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,MerritCR/merrit,MerritCR/merrit,WANdisco/gerrit,joshuawilson/merrit,MerritCR/merrit,GerritCodeReview/gerrit