Allows to fire VKEY_UNKNOWN key events.
On Linux, we have keys with keycode=VKEY_UNKNOWN, for example, ISO_LEVEL3_LATCH key. We have to emit keydown/up events appropriately even if keycode=VKEY_UNKNOWN.
Just FYI: Firefox also fires keyevents with keycode=0, so the behavior will be compatible. Compared to IE and Chrome on Windows, it's still compatible because there is no such key as ISO_LEVEL3_LATCH, etc. on Windows.
BUG=402320
TEST=Done manually.
Review URL: https://codereview.chromium.org/
665863002
Cr-Commit-Position: refs/heads/master@{#300438}