Add experimental Support for DOM3 KeyboardEvent key value

public/web/WebInputEvent.h

Index: public/web/WebInputEvent.h
diff --git a/public/web/WebInputEvent.h b/public/web/WebInputEvent.h
index c9e73c03e601bcf2e32b372c05ea2c2ca29eea13..173f6140630759b29cf1b5cc6c1f0316ad815fec 100644
--- a/public/web/WebInputEvent.h
+++ b/public/web/WebInputEvent.h
@@ -247,6 +247,10 @@ public:
     // code enum are defined in ui/events/keycodes/dom4/keycode_converter_data.h.
     int domCode;
 
+    // The DOM key enum of the key pressed as passed by the embedder. DOM

[Wez, 2015/03/04 23:00:26]

This (and the above domCode field) are embedder enum values representing the DOM
|key| and |code| of the key. As noted elsewhere, we shouldn't refer to the
Chromium code locations to document these - just mention that the values are
opaque to Blink, controlled purely by the embedder (i.e. by Chromium).

[Habib Virji, 2015/03/09 15:14:27]

As previously stated, it was added as rbyers suggested. Please suggest if to
remove. 

[Rick Byers, 2015/04/20 19:49:46]

I don't have a strong preference.  It's not like there is really any hard
separation between chromium and blink anymore (especially with the repos
merging) so I don't see any realistic downside to specifying chromium paths to
help developers understand what's going on (DEPS rules will prevent you from
including chromium headers into blink, so I don't think there's a risk of
developers doing dumb things with this knowledge).  But if wez@ feels strongly,
then I'm OK being vague as well.

[Habib Virji, 2015/04/29 16:00:50]

@wez: Do you have a suggestion on this?

[Rick Byers, 2015/04/29 17:34:26]

Since we haven't heard from wez@ on this yet, I suggest you just land it the way
it is.  But if you prefer you can take the comment out.  It's a minor issue, not
worth delaying your CL for further.

+    // key enum are defined in ui/events/keycodes/dom3/dom_key_data.h
+    int domKey;
+
     // This identifies whether this event was tagged by the system as being
     // a "system key" event (see
     // http://msdn.microsoft.com/en-us/library/ms646286(VS.85).aspx for