The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6a6ca375d312e822a65ee997443ed7bc316cc310

commit 6a6ca375d312e822a65ee997443ed7bc316cc310
Author: Balazs Engedy <engedy@chromium.org>
Date: Fri Sep 14 13:04:45 2018

Adjust transport list icon sizes and padding.

Bug:  881321 
Change-Id: Ie6b3476a9c8dc14734f545b6dbe7cc8f6b85ca03
Reviewed-on: https://chromium-review.googlesource.com/1221324
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Jun Choi <hongjunchoi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591331}
[modify] https://crrev.com/6a6ca375d312e822a65ee997443ed7bc316cc310/chrome/browser/ui/views/webauthn/authenticator_request_dialog_view.cc
[modify] https://crrev.com/6a6ca375d312e822a65ee997443ed7bc316cc310/chrome/browser/ui/views/webauthn/authenticator_request_sheet_view.cc
[modify] https://crrev.com/6a6ca375d312e822a65ee997443ed7bc316cc310/chrome/browser/ui/views/webauthn/transport_list_view.cc
[modify] https://crrev.com/6a6ca375d312e822a65ee997443ed7bc316cc310/chrome/browser/ui/webauthn/other_transports_menu_model.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/846f77b51de1d44a72171c83387b2f8cb2429ac4

commit 846f77b51de1d44a72171c83387b2f8cb2429ac4
Author: Balazs Engedy <engedy@chromium.org>
Date: Fri Sep 14 16:28:16 2018

WebAuthn: Add drop-down arrow icon to `Choose another option`.

Bug:  881321 
Change-Id: I6f4d2ad1ae27f8dbb6f85e24442cf3e25287a7bc
TBR: sky@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/1209746
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Jun Choi <hongjunchoi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591367}
[modify] https://crrev.com/846f77b51de1d44a72171c83387b2f8cb2429ac4/chrome/browser/ui/views/webauthn/authenticator_request_dialog_view.cc
[modify] https://crrev.com/846f77b51de1d44a72171c83387b2f8cb2429ac4/ui/views/BUILD.gn
[add] https://crrev.com/846f77b51de1d44a72171c83387b2f8cb2429ac4/ui/views/vector_icons/menu_drop_arrow.icon

Requesting merging these cosmetic changes to address comments from UX review. The changes already went through two Canaries and are very simple in nature.

This bug requires manual review: M70 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: benmason@(Android), kariahda@(iOS), geohsu@(ChromeOS), abdulsyed@(Desktop)

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

FYI, this affects all desktop platforms.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4057bc920d239cc9a2cfd22e3e78a5e3e24b6ad9

commit 4057bc920d239cc9a2cfd22e3e78a5e3e24b6ad9
Author: Balazs Engedy <engedy@chromium.org>
Date: Tue Sep 18 13:18:39 2018

Adjust transport list icon sizes and padding.

Bug:  881321 
Change-Id: Ie6b3476a9c8dc14734f545b6dbe7cc8f6b85ca03
Reviewed-on: https://chromium-review.googlesource.com/1221324
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Jun Choi <hongjunchoi@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#591331}(cherry picked from commit 6a6ca375d312e822a65ee997443ed7bc316cc310)
Reviewed-on: https://chromium-review.googlesource.com/1230029
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/branch-heads/3538@{#487}
Cr-Branched-From: 79f7c91a2b2a2932cd447fa6f865cb6662fa8fa6-refs/heads/master@{#587811}
[modify] https://crrev.com/4057bc920d239cc9a2cfd22e3e78a5e3e24b6ad9/chrome/browser/ui/views/webauthn/authenticator_request_dialog_view.cc
[modify] https://crrev.com/4057bc920d239cc9a2cfd22e3e78a5e3e24b6ad9/chrome/browser/ui/views/webauthn/authenticator_request_sheet_view.cc
[modify] https://crrev.com/4057bc920d239cc9a2cfd22e3e78a5e3e24b6ad9/chrome/browser/ui/views/webauthn/transport_list_view.cc
[modify] https://crrev.com/4057bc920d239cc9a2cfd22e3e78a5e3e24b6ad9/chrome/browser/ui/webauthn/other_transports_menu_model.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fe6302eaa0c4e303d2aae89e425f7b19c20e247b

commit fe6302eaa0c4e303d2aae89e425f7b19c20e247b
Author: Balazs Engedy <engedy@chromium.org>
Date: Tue Sep 18 13:19:30 2018

WebAuthn: Add drop-down arrow icon to `Choose another option`.

Bug:  881321 
Change-Id: I6f4d2ad1ae27f8dbb6f85e24442cf3e25287a7bc
TBR: sky@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/1209746
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Jun Choi <hongjunchoi@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#591367}(cherry picked from commit 846f77b51de1d44a72171c83387b2f8cb2429ac4)
Reviewed-on: https://chromium-review.googlesource.com/1230065
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/branch-heads/3538@{#488}
Cr-Branched-From: 79f7c91a2b2a2932cd447fa6f865cb6662fa8fa6-refs/heads/master@{#587811}
[modify] https://crrev.com/fe6302eaa0c4e303d2aae89e425f7b19c20e247b/chrome/browser/ui/views/webauthn/authenticator_request_dialog_view.cc
[modify] https://crrev.com/fe6302eaa0c4e303d2aae89e425f7b19c20e247b/ui/views/BUILD.gn
[add] https://crrev.com/fe6302eaa0c4e303d2aae89e425f7b19c20e247b/ui/views/vector_icons/menu_drop_arrow.icon

Unable to verify this issue on Linux and Windows as we are not seeing Choose other option button in both OS. 

Attaching screen-cast for reference.

@engedy: Could you please look into the screen-cast and let us know if anything is missed from our end. Please help in verifying the fix.

Thanks! 

Deleted: Linux_No option seen.webm 3.8 MB

	Linux_No option seen.webm 3.8 MB View Download

You could start Chrome with the experimental --enable-features=WebAuthenticationBle flag, which also enables Bluetooth security keys, which will not always work yet, but at least will show the drop-down on the UI.