Fix incorrect overlayScrollbarsEnabled value on Android.

In https://chromiumcodereview.appspot.com/23513028, I changed
the layout test harness to always set
RuntimeEnabledFeatures::overlayScrollbarsEnabled to false between
tests, but on Android, the original value is true.

Fix this by moving internals.setOverlayScrollbarsEnabled to
internals.settings.setOverlayScrollbarsEnabled and backing up and
restoring the value between tests.

BUG=290505

Review URL: https://chromiumcodereview.appspot.com/23616004

git-svn-id: svn://svn.chromium.org/blink/trunk@157786 bbb929c8-8fbe-4397-9dbb-9b2b20218538