Fix RTL scroll bars being misrendered on Android.

In https://chromiumcodereview.appspot.com/23734003 , I tried to
include information about whether to use a left side scrollbar by
adding a textDirection method to WebScrollbar.  It missed some cases
when there is no horizontal scrolling.

I also renamed to textDirection() and isRightToLeft() to
shouldPlaceVerticalScrollbarOnLeft() because writing mode direction can
also cause the scrollbar to be on the left.

Fix the bug in ScrollbarThemeOverlay::paintThumb to take the left side
scrollbars into account.

BUG=274010

Review URL: https://chromiumcodereview.appspot.com/23480037

git-svn-id: svn://svn.chromium.org/blink/trunk@157406 bbb929c8-8fbe-4397-9dbb-9b2b20218538