The cause is a little unclear, but the stdio mentions ConcurrentModificationException.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1079d8a721465cf5626bfe0f1e4646eaad644829

commit 1079d8a721465cf5626bfe0f1e4646eaad644829
Author: johnme <johnme@chromium.org>
Date: Mon Jan 23 15:51:15 2017

Mark UrlManagerTest#testAddUrlGarbageCollectsForSize flaky

BUG= 683923 
TBR=cco3@chromium.org,nyquist@chromium.org
NOTRY=true

Review-Url: https://codereview.chromium.org/2652703002
Cr-Commit-Position: refs/heads/master@{#445382}

[modify] https://crrev.com/1079d8a721465cf5626bfe0f1e4646eaad644829/chrome/android/javatests/src/org/chromium/chrome/browser/physicalweb/UrlManagerTest.java

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c2c9e69eb418629cc7f2ee5a60d913690883145e

commit c2c9e69eb418629cc7f2ee5a60d913690883145e
Author: johnme <johnme@chromium.org>
Date: Mon Jan 23 16:43:58 2017

Mark UrlManagerTest#testAddUrlGarbageCollectsForSize flaky

(reland of https://codereview.chromium.org/2652703002 which
was reverted in https://codereview.chromium.org/2654503002)

BUG= 683923 
TBR=cco3@chromium.org,nyquist@chromium.org

Review-Url: https://codereview.chromium.org/2650713003
Cr-Commit-Position: refs/heads/master@{#445388}

[modify] https://crrev.com/c2c9e69eb418629cc7f2ee5a60d913690883145e/chrome/android/javatests/src/org/chromium/chrome/browser/physicalweb/UrlManagerTest.java

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9bbc684353acc634cb47b87bc945d0e5158841f1

commit 9bbc684353acc634cb47b87bc945d0e5158841f1
Author: mattreynolds <mattreynolds@chromium.org>
Date: Mon Jan 30 21:18:28 2017

Ensure message listeners invoke UrlManager on UI thread

To avoid concurrency issues, all calls to UrlManager must be made on the
main thread. This CL touches the BackgroundMessageListener in
PhysicalWebBleClient.

BUG= 683923 

Review-Url: https://codereview.chromium.org/2658573006
Cr-Commit-Position: refs/heads/master@{#447066}

[modify] https://crrev.com/9bbc684353acc634cb47b87bc945d0e5158841f1/chrome/android/java/src/org/chromium/chrome/browser/physicalweb/PhysicalWebBleClient.java

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9bbc684353acc634cb47b87bc945d0e5158841f1

commit 9bbc684353acc634cb47b87bc945d0e5158841f1
Author: mattreynolds <mattreynolds@chromium.org>
Date: Mon Jan 30 21:18:28 2017

Ensure message listeners invoke UrlManager on UI thread

To avoid concurrency issues, all calls to UrlManager must be made on the
main thread. This CL touches the BackgroundMessageListener in
PhysicalWebBleClient.

BUG= 683923 

Review-Url: https://codereview.chromium.org/2658573006
Cr-Commit-Position: refs/heads/master@{#447066}

[modify] https://crrev.com/9bbc684353acc634cb47b87bc945d0e5158841f1/chrome/android/java/src/org/chromium/chrome/browser/physicalweb/PhysicalWebBleClient.java

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9bbc684353acc634cb47b87bc945d0e5158841f1

commit 9bbc684353acc634cb47b87bc945d0e5158841f1
Author: mattreynolds <mattreynolds@chromium.org>
Date: Mon Jan 30 21:18:28 2017

Ensure message listeners invoke UrlManager on UI thread

To avoid concurrency issues, all calls to UrlManager must be made on the
main thread. This CL touches the BackgroundMessageListener in
PhysicalWebBleClient.

BUG= 683923 

Review-Url: https://codereview.chromium.org/2658573006
Cr-Commit-Position: refs/heads/master@{#447066}

[modify] https://crrev.com/9bbc684353acc634cb47b87bc945d0e5158841f1/chrome/android/java/src/org/chromium/chrome/browser/physicalweb/PhysicalWebBleClient.java

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/clank/internal/apps/+/24d79bf8ddf913ab29e675cae14d7e05ecc732be

commit 24d79bf8ddf913ab29e675cae14d7e05ecc732be
Author: Matt Reynolds <mattreynolds@google.com>
Date: Fri Jan 27 00:27:28 2017

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/clank/internal/apps/+/24d79bf8ddf913ab29e675cae14d7e05ecc732be

commit 24d79bf8ddf913ab29e675cae14d7e05ecc732be
Author: Matt Reynolds <mattreynolds@google.com>
Date: Fri Jan 27 00:27:28 2017

We'd like to merge these changes for M57:

https://codereview.chromium.org/2658573006

https://chrome-internal.googlesource.com/clank/internal/apps/+/24d79bf8ddf913ab29e675cae14d7e05ecc732be

Your change meets the bar and is auto-approved for M57. Please go ahead and merge the CL to branch 2987 manually. Please contact milestone owner if you have questions.
Owners: amineer@(clank), cmasso@(bling), ketakid@(cros), govind@(desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

The fix is in master and cherry-picked to M57 branch (2987).

Issue 684674 has been merged into this issue.

This issue has been approved for a merge. Please merge the fix to any appropriate branches as soon as possible!

If all merges have been completed, please remove any remaining Merge-Approved labels from this issue.

Thanks for your time! To disable nags, add the Disable-Nags label.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot