After Aura copied the original Linux scrollbar behavior, it never got updated
with Windows-specific native behaviors from ScrollbarThemeWin.cpp:

* Snap the scrollbar back to its drag origin when dragged too far off the track.
  This code matches the old code we had for years and also matches my Win 7
  native scrollbars pixel-for-pixel.

* Don't jump the scroll thumb to the mouse cursor on a middle-click, only on a
  shift-click.  Middle-click must be a GTK-ism.

This is the Blink side of the changes to re-add this behavior.

BUG=337919
TEST=Drag a scroll thumb to somewhere in the track, then without releasing, move the mouse far away from the track.  The scroll thumb should snap back to the scroll origin.
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/151483008

git-svn-id: svn://svn.chromium.org/blink/trunk@168177 bbb929c8-8fbe-4397-9dbb-9b2b20218538