Implement KeyboardEvent.repeat attribute

Implement KeyboardEvent.repeat attribute as per the DOM level 3 specification:
https://dvcs.w3.org/hg/dom3events/raw-file/tip/html/DOM3-Events.html#widl-KeyboardEvent-repeat

This attribute is already implemented in IE10 and is currently being worked on
for Firefox.

BUG=263724
TEST=fast/events/constructors/keyboard-event-constructor.html
TEST=fast/events/keyboardevent-repeat.html

Review URL: https://codereview.chromium.org/23444080

git-svn-id: svn://svn.chromium.org/blink/trunk@158868 bbb929c8-8fbe-4397-9dbb-9b2b20218538