Google Chrome	65.0.3325.209 (Official Build) (64-bit)
Platform	10323.67.0 (Official Build) stable-channel panther

What steps will reproduce the problem?
(1) In a chrome.input.ime.onKeyEvent listener, log `key` property of incoming events to the console.
(2) Install the IME extension from (1) and activate its input method.
(3) In chrome://settings/keyboard-overlay, enable “Treat top-row keys as function keys”.
(4) Press a function key.


What is the expected result?
A keyboard event with a key value of the form 'F1' (per https://www.w3.org/TR/uievents-key/#keys-function).


What happens instead?
A keyboard event with a ChromeOS-specific key value such as 'HistoryBack', or the string '\u0000' for F11 and F12.