From the Mac side, Fn-Up/Down support seems to be universal for replacing the missing Page Up / Page Down keys. I bet Chrome is Ctrl-Alt-Fn-Down, copying Mac:

Down - Cursor trajectory

Fn - Page instead of Line

Command (Alt) - Document instead of Page

Shift - Text selection mode

(But I had to hit the keys and then look at my hands to figure out what they were, because I'm just used to keeping the modifier layers in muscle memory, so I could be wrong.)