GitLab

This bug would only affect the non-root-layer-scrolling path and only
when the page is scrolled. This CL fixes the issue and cleans up the
surrounding code a bit.

I also took the opportunity to cleanup the drag-and-drop autoscrolling
tests:
 - Modernized using testharness.js and new test style guidelines
 - I've based them all on the drag-and-drop-autoscroll-frame.js
   script for commonality.
 - All the tests now occur at a non-0 scroll offset to make sure we
   catch these kinds of coordinate space bugs.

Change-Id: I3191796917f23b2e9b2cc3f561813176fa2dec9a
Reviewed-on: https://chromium-review.googlesource.com/972148
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Sandra Sun <sunyunjia@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545076}