Define DOM_KEY_LOCATION_* constants on KeyboardEvent

Define DOM_KEY_LOCATION_* constants on the KeyboardEvent interface to match
the latest specification:
http://www.w3.org/TR/DOM-Level-3-Events/#events-KeyboardEvent

Those constants are already supported by IE10 and Firefox 22.

Note that the following constants are not supported internally yet:
DOM_KEY_LOCATION_MOBILE and DOM_KEY_LOCATION_JOYSTICK (crbug.com/265446).

BUG=263724

Review URL: https://chromiumcodereview.appspot.com/20986003

git-svn-id: svn://svn.chromium.org/blink/trunk@155231 bbb929c8-8fbe-4397-9dbb-9b2b20218538