Is this a flaky failure? We should be catching this in the bots.

Since you've set this as "ExternalDependency" -- is there a bug tracking this in another repo? If so, please link to it here so we can keep track.

Previously, we handled the error by manually setting roles (after some discussion w/a11y and Polymer folks): 591552.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1a7ae6b085e411ec9be7169fce3c5fe9db2b6c76

commit 1a7ae6b085e411ec9be7169fce3c5fe9db2b6c76
Author: takumif <takumif@chromium.org>
Date: Mon Jun 27 20:09:17 2016

Add role="presentation" to Media Router #search-results element

To address the ARIA_08 accessibility issue [1], we added
the attribute role="presentation" to certain HTML elements [2].
However, the attribute was removed from one of the elements in
a later commit [3], so this CL adds the attribute back.

This issue surfaced while I was writing a test for the search result
list view [4].

[1] http://crbug/591552
[2] https://codereview.chromium.org/1849283004
[3] https://codereview.chromium.org/1754713005
[4] https://codereview.chromium.org/2096733002

BUG= 623260 
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2100833003
Cr-Commit-Position: refs/heads/master@{#402266}

[modify] https://crrev.com/1a7ae6b085e411ec9be7169fce3c5fe9db2b6c76/chrome/browser/resources/media_router/elements/media_router_container/media_router_container.html