Pressing X closes my chrome tab (MacOS)
Check if a shortcut is enabled for your 'x' key from your Mac's settings, that would be in Preferences→Keyboard→Shortcuts→App Shortcuts→Google Chrome and remove it if it exists there.
Do you happen to have Vimium installed? I found that it was overwriting lowercase x.
If you open Vimium's options, you can add custom key mappings. I did:
unmap x