Fix page jumping back to initial position after an overflow-bounce scroll.

This CL is forked from the CL at https://codereview.chromium.org/122093002,
original author thakis. This CL fixes the layout test to correctly
differentiate between the correct and incorrect behaviors.

thakis's CL in turn is a merge of http://trac.webkit.org/changeset/160791 and
http://trac.webkit.org/changeset/160898 from Beth Dakin <bdakin@apple.com>;.

Depends on Chromium CL: https://codereview.chromium.org/283713002/

BUG=330887

Review URL: https://codereview.chromium.org/282743002

git-svn-id: svn://svn.chromium.org/blink/trunk@174427 bbb929c8-8fbe-4397-9dbb-9b2b20218538