Add the text direction to the WebScrollbar interface.

When drawing thinning scrollbars, we'll need this information.
The text direction is always LTR for horizontal scrollbars.

BUG=None
TEST=None

Review URL: https://chromiumcodereview.appspot.com/23734003

git-svn-id: svn://svn.chromium.org/blink/trunk@156888 bbb929c8-8fbe-4397-9dbb-9b2b20218538