see https://codereview.chromium.org/1947533003

Space activates on up, enter activates on down. This already worked for toolbar buttons, and it was fixed for location bar icons. It should also be fixed for browser action buttons.

Implementation note: browser action buttons currently are a specialization of views::MenuButton which has different activation behavior. They look and act more like normal buttons so should probably inherit from ToolbarButton. Note that browser actions have an attached menu, shown when you right click. ToolbarButtons do support showing an attached menu (e.g. the recents menu on the back button) so this should not be too difficult to manage.