Bindings for CSSOM View smooth scroll API

This adds custom bindings for Window.scroll, Window.scrollTo
Window.scrollBy, Element.scrollLeft, and Element.scrollTop,
needed for the CSSOM View smooth scroll API.

More specifically:
1) An optional ScrollOptions argument is added to
Window.scroll, Window.scrollTo, and Window.scrollBy.
2) The setter for Element.scrollLeft can now take a
ScrollOptionsHorizontal dictionary instead of an int.
3) The setter for Element.scrollRight can now take a
ScrollOptionsVertical dictionary instead of an int.

These behavior changes are protected by a runtime flag.

Spec: http://www.w3.org/TR/cssom-view/

Intent-to-implement:
https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/YLWs9N46XjQ/mK7rCJ26BqAJ


BUG=243871

Review URL: https://codereview.chromium.org/146003002

git-svn-id: svn://svn.chromium.org/blink/trunk@166322 bbb929c8-8fbe-4397-9dbb-9b2b20218538