Maybe allow remapping of keys. Do to the lack of ChromeOS keyboards I think this key remapping should be a must. 

Tried testing the issue on reported chrome version #69.0.3497.100using Windows 10 by following below steps.

Steps:
=====
1.Launched chrome.
2.Using an external keyboard and pressed keys Ctrl + Shift + window + F5.
3.Could not view any  "allow for a print screen selection of the screen". 
Note: Followed same procedure with laptop internal keyboard, then also could not observe any " allow for a print screen selection of the screen. ".

@reporter: Could you please let us know if anything is being missed here. Request you retry the issue on latest stable and let us know if the issue still persists. You can download the latest stable from-"https://www.chromium.org/getting-involved/dev-channel" 
Thanks.!

Sadly are using Mac keyboards. The laptop works great pressing the combination, but not the external keyboard. I tried every combination with no success. 

Laptop worked - Ctrl - Shift - F5   (screen grays out and allow for selection to print screen)

External Keybaord (mac) -  Ctrl - Shift - F5  (nothing)
                            Ctrl - Shift - option - F5 (nothing)
                           Ctrl - Shift - command - F5 (nothing) 

Tasted on 69, and also 72 dev. 

Update: Based on this chart there is no print screen map for windows or mac external keyboards. 

https://defkey.com/chromebook-and-chrome-os-shortcuts

Thank you for providing more feedback. Adding the requester to the cc list.

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

***Mass UI Triage **

Unfortunately test team do not have External Keybaord (mac)for trying a repro. Keeping the bugs open of expert review.