Start EditableCombobox.

See associated bug for design doc and screen recording showing the new

This CL gets us most of the way there, but there are still things to do
in subsequent CLs before we start using this control:
- I only tested this on Mac for now. I will add support for other OSes
- The accessibility isn't quite right. It says to select with
control+option+space, but that doesn't work. I noticed that it doesn't
work for the regular Combobox either, so I will fix it for both in a
later CL.

Bug: 923660
Change-Id: I8e959e068af23491a842c4bc0fa9661cbd2869dd
Commit-Queue: Edin Kadric <>
Reviewed-by: Scott Violet <>
Reviewed-by: Vasilii Sukhanov <>
Cr-Commit-Position: refs/heads/master@{#636033}
13 files changed