Add support for KeyboardEvent.getModifierState()

Add support for KeyboardEvent.getModifierState() as per the latest specification:
http://www.w3.org/TR/DOM-Level-3-Events/#events-KeyboardEvent-getModifierState

Note that only the following modifiers are currently supported (crbug.com/265458):
"Control", "Alt", "Shift", "Meta".

BUG=263724

Review URL: https://chromiumcodereview.appspot.com/21030004

git-svn-id: svn://svn.chromium.org/blink/trunk@155158 bbb929c8-8fbe-4397-9dbb-9b2b20218538