How to Slave Chromebook's Display for Android by RB/HDMI? Condition: Slave Chromebook's Display for Android by Microcontroller/HDMI
Proposal: Microcontroller as a router in the local network which streams phone output to the local network, which Chromebook can access and view real-time
Support: Chromebook's display cannot work as a real slave itself for the phone   

I would like to use my Android phone on the external monitor. I am thinking if any Chromebook's monitor could work as a slave for your Android phone, although it is not supported by their design. A microcontroller is needed in the solution between Chromebook and Android phone. 

Hardware: Chromebook 2 Toshiba 13", Chromebook 2 Acer 13" and/or any
Phone: Samsung Galaxy S4 gt-i9506 and/or any
Phone System: CyanogenMod 13, Temasek's rom with patches
Microcontroller open: Raspberry Pi 3 b (RB3b), FPGA (SnickerDoodle), Pine64
Network: localhost HTTPS
You cannot do this without changes in the Chromebook itself.