Add initial Screen Orientation API support: IDL changes

Add initial Screen Orientation API support: IDL changes:
https://dvcs.w3.org/hg/screen-orientation/raw-file/tip/Overview.html

This new API is implemented behind a "ScreenOrientation" runtime-enabled flag.
The intent to implement was sent to blink-dev:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/lgUq8K9IGlU

This CL changes Screen to be an EventTarget. While the Screen Orientation API
has been implemented behind a RuntimeEnabled flag, there is no way of having
inheritance depend on the flag's value. The Web exposed effect of this is that
people will now be able to dispatch custom events on the window.screen
interface, which in itself is harmless.

R=abarth
BUG=162827
TEST=screen_orientation/screenorientation-api.html

Review URL: https://codereview.chromium.org/168763002

git-svn-id: svn://svn.chromium.org/blink/trunk@167259 bbb929c8-8fbe-4397-9dbb-9b2b20218538