Remove the use of KeyEvent.keyIdentifier.
We should use KeyboardEvent.key instead.
For the shortcut identifier of cr.ui.Command (<command> in main.html), we still depends on keyIdentifier, but it should be handled in a separate CL.
BUG=
612717
TEST=run browser_tests
Committed:
https://crrev.com/9e5d27298cb447dbf20cbcd189dfb36a4d94960d
Cr-Commit-Position: refs/heads/master@{#398876}