Add 'code' of DOM UIEvents field to ui::KeyEvent.

This patch enables ui::KeyEvent contain a field representing the physical location in the keyboard, which can be converted to scan code, USB HID Usage, etc.

UIEvents:
http://www.w3.org/TR/uievents/

Mapping between platform dependent scan code and USB HID Usage:
ui/base/keycodes/keycode_converter_data.h.


BUG=312218

Review URL: https://codereview.chromium.org/58003004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237789 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237789