Use absolute paths in Polymer tests

JS tests now support source-absolute paths.
This makes Polymer tests a little simpler, since they no longer
need to rewrite the includes in polymer_browser_test_base to be
relative to the current directory.

This CL contains 2 changes:
1. (Start here):
polymer_browser_test_base.js no longer has a function getLibraries,
which was where the paths were previously rewritten

2. All the Polymer tests no longer call getLibraries, since they
no longer need to rewrite the paths. They now just append
super.extraLibraries if they need to, or leave it as-is if they
don't.
Polymer tests no longer need a variable ROOT_PATH.

Change-Id: I43da00a8c678b48686df7f1a390d4d14e3b435b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1583880
Commit-Queue: A Olsen <olsen@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654061}
30 files changed