Chrome Version: 59.0.3071.36 (Official Build)0a97305f0587704e901b20cc4060c997d6c4eaff-refs/branch-heads/3071@{#365} 32/64-bit.
OS: Windows (7,8,8.1,10),Linux (14.04 LTS),Mac OS X(10.11.6,10.12.1).

What steps will reproduce the problem?
1. Launch chrome, open 'DevTools' and go 'Settings' page.
2. Go to 'Blackboxing' section and click on 'Add pattern' button.
3. Press 'Tab' key such that blue focus reaches to 'Behavior' dropdown list.
4. Now, hit 'Enter' key from keyboard and observe.

Nothing happens on hitting 'Enter' key from keyboard.
'Behavior' dropdown list should get opened on hitting 'Enter' key.

This is a non-regression issue, seen from M-48 series.

Note: 
1. The same issue is reproducible for 'Devices' dropdown list under 'Devices' section.
2. Able to open all the dropdown list (For Ex: Theme, Panel layout, Color format etc.) under 'Preferences' section using 'Enter' key.

Kindly review the attached scree-cast for reference.
 

Deleted: Actual behavior.mp4 736 KB

	Actual behavior.mp4 736 KB View Download