Extensions WebUI: Fix a11y tests to work with Polymer 2.

Basically eliminating usage of /deep/ selectors.

Bug: 875449,860069
Change-Id: I7e53055b712ee6701f240ba6f50c9eb1918d88b1
Reviewed-on: https://chromium-review.googlesource.com/1233113
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592474}
2 files changed