ScrollView::windowClipRect shouldn't take a raw bool

Rather than passing a bool and converting it to an enum, we should just pass an
enum. Making this change required moving the enum into the global scope, which
meant I needed to touch a bunch of files to move the name outside of the
namespace.

Review URL: https://codereview.chromium.org/171813009

git-svn-id: svn://svn.chromium.org/blink/trunk@167483 bbb929c8-8fbe-4397-9dbb-9b2b20218538