This is a partial roll-forward of c/106190
This replaces all loads of iron-test-helpers with a load of a file
that wraps it, and adds that file to test files that do not currently
load iron-test-helpers.
A future CL will also install polymer-resin via common-test-helpers.html.
I tested by running
$ WCT_ARGS="-l chrome" ./polygerrit-ui/app/run_test.sh
Change-Id: Ifb3cd2c8db13d724f57e56e7e78045470d103a43