Fractional scroll support for main thread document scrolling

This is the first step to make main thread fractional scrolling. This CL
replaces the scroll position and scroll offset in some key steps of document
scrolling from IntPoint/IntSize to DoublePoint/DoubleSize.

BUG=414283

Review URL: https://codereview.chromium.org/595743003

git-svn-id: svn://svn.chromium.org/blink/trunk@183089 bbb929c8-8fbe-4397-9dbb-9b2b20218538