Add ScrollOptions IDL dictionary

To remove generic Dictionary from DOMWindow.idl.
ScrollOptions isn't a spec-defined IDL dictionary, but we want to
remove direct dependency on Dictionary from core, because the current
Dictionary(Helper) is hard to maintain and we are planning to
do refactoring. Direct dependency on Dictionary from core/modules
prevents this refactoring.

Tests under fast/dom/Window cover this change.

BUG=403150

Review URL: https://codereview.chromium.org/565863002

git-svn-id: svn://svn.chromium.org/blink/trunk@182545 bbb929c8-8fbe-4397-9dbb-9b2b20218538