After Aura copied the original Linux scrollbar behvaior, 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 Chrome 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=avi@chromium.org

Review URL: https://codereview.chromium.org/175903002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254305 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254305