Seems like the test times out. Some logging from the console looks suspicious:

[12155:12155:0913/205537.677921:INFO:CONSOLE(5766)] "Initialized all volumes.", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/background_common_scripts.js (5766)
[12155:12155:0913/205547.192524:INFO:CONSOLE(3773)] "Uncaught (in promise) Error: Assertion failed: Missing required element: bottom-toolbar", source: chrome-extension://nlkncpkkdoccmpiclbokaimcnedabhhm/js/gallery_scripts.js (3773)
[12155:12155:0913/205547.231013:INFO:CONSOLE(5030)] "Received the result of openGallery", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/background_common_scripts.js (5030)
[12155:12155:0913/205550.445104:INFO:CONSOLE(156)] "at RemoteCallGallery.RemoteCall.waitForElement (/remote_call.js:182:15): Element .gallery[mode="slide"] is not found.", source: chrome-extension://ejhcmmdhhpdhhgmifplfmjobgegbibkn/test_util.js (156)
[12155:12155:0913/205553.423338:INFO:CONSOLE(156)] "at RemoteCallGallery.RemoteCall.waitForElement (/remote_call.js:182:15): Element .gallery[mode="slide"] is not found.", source: chrome-extension://ejhcmmdhhpdhhgmifplfmjobgegbibkn/test_util.js (156)
[12155:12155:0913/205556.366950:INFO:CONSOLE(156)] "at RemoteCallGallery.RemoteCall.waitForElement (/remote_call.js:182:15): Element .gallery[mode="slide"] is not found.", source: chrome-extension://ejhcmmdhhpdhhgmifplfmjobgegbibkn/test_util.js (156)
[12155:12155:0913/205559.324005:INFO:CONSOLE(156)] "at RemoteCallGallery.RemoteCall.waitForElement (/remote_call.js:182:15): Element .gallery[mode="slide"] is not found.", source: chrome-extension://ejhcmmdhhpdhhgmifplfmjobgegbibkn/test_util.js (156)
[1

Hmm, this test has been flaky for quite a while: https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webui_polymer2_browser_tests&showAllRuns=true&tests=TraverseSlideImagesOnDownloads

Probably just going to disable this test.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f8a009872074e809a9d4b874b91fdbe55b6652f2

commit f8a009872074e809a9d4b874b91fdbe55b6652f2
Author: Darren Shen <shend@chromium.org>
Date: Mon Sep 17 01:44:08 2018

[sheriff] Disable two flaky GalleryBrowserTest tests.

- GalleryBrowserTest.TraverseSlideImagesOnDownloads
- GalleryBrowserTest.OpenMultipleImagesOnDownloads

Both are flaky on Chromium OS MSAN and ASAN.

TBR=noel@chromium.org

Bug:  884399 ,  884404 
Change-Id: I0b15aadc1274f2aac5b295d87176fb059464e4d1
Reviewed-on: https://chromium-review.googlesource.com/1226729
Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591607}
[modify] https://crrev.com/f8a009872074e809a9d4b874b91fdbe55b6652f2/chrome/browser/chromeos/file_manager/gallery_browsertest.cc

Hi Trent, can you take a look when you get back (assuming it's relevant to your work).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cb0f8883538d9c7cf386d7305d0f2da5200964e9

commit cb0f8883538d9c7cf386d7305d0f2da5200964e9
Author: Darren Shen <shend@chromium.org>
Date: Mon Sep 17 04:36:38 2018

Revert "[sheriff] Disable two flaky GalleryBrowserTest tests."

This reverts commit f8a009872074e809a9d4b874b91fdbe55b6652f2.

Reason for revert: Browser tests weren't failing, just polymer_webui tests.

Original change's description:
> [sheriff] Disable two flaky GalleryBrowserTest tests.
> 
> - GalleryBrowserTest.TraverseSlideImagesOnDownloads
> - GalleryBrowserTest.OpenMultipleImagesOnDownloads
> 
> Both are flaky on Chromium OS MSAN and ASAN.
> 
> TBR=noel@chromium.org
> 
> Bug:  884399 ,  884404 
> Change-Id: I0b15aadc1274f2aac5b295d87176fb059464e4d1
> Reviewed-on: https://chromium-review.googlesource.com/1226729
> Reviewed-by: Darren Shen <shend@chromium.org>
> Commit-Queue: Darren Shen <shend@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#591607}

TBR=shend@chromium.org

Change-Id: I715cfc53aa6d41874b26e020ba714963a56f55c5
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug:  884399 ,  884404 
Reviewed-on: https://chromium-review.googlesource.com/1227858
Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591612}
[modify] https://crrev.com/cb0f8883538d9c7cf386d7305d0f2da5200964e9/chrome/browser/chromeos/file_manager/gallery_browsertest.cc