Revert "Scroll bars continue to scroll when finger leaves the scroll bar."

This reverts commit ed0a05ecf6e4a8efe81494a390cb8c322f319d65.

This patch seems to cause breakge in two tests under Aura on ChromeOS,
Windows, and Linux:

* WebContentsViewAuraTest.RepeatedQuickOverscrollGestures
* WebContentsViewAuraTest.OverscrollScreenshot

See [1], [2], and [3] for examples.

[1]: http://build.chromium.org/p/tryserver.chromium/builders/linux_chromeos/builds/181659
[2]: http://build.chromium.org/p/tryserver.chromium/builders/win_rel/builds/232710
[3]: http://build.chromium.org/p/tryserver.chromium/builders/linux_aura/builds/103752

BUG=324479
TBR=tdresser@chromium.org
NOTRY=true
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/102663003

git-svn-id: svn://svn.chromium.org/blink/trunk@163414 bbb929c8-8fbe-4397-9dbb-9b2b20218538