Fix Tab to switch focus instead of inserting \t.

Blink uses the WebKeyboardEvent::keyIdentifier string to detect
Tab key presses. Make sure that we properly populate this field
before forwarding the event.

Change-Id: I7da52b5678cfc1bd7f433d9cc9cd4531e6a24728
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>