<third_party/WebKit> Fix key event crash on Windows.

This fixes the ASSERT that was triggerd on Windows in
PlatformKeyboardEvent::disambiguateKeyDownEvent  because
we do our own conversion from native events to web events.

This is the same way it was handled in the Qt specific
PlatformKeyboardEvent in WebKit.

Change-Id: I177c238bde1e0101b6abe9d5bbe3ba2e47908f1a
Reviewed-by: Andras Becsi <andras.becsi@digia.com>