Reduce code duplication in RenderListMarker

Change the suffix/space metric and painting logic in RenderListMarker::
paint to reuse code for the LTR and RTL cases. As a part of this change
the hardcoded direction for constructTextRun can also be removed.

BUG=333004
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/141803013

git-svn-id: svn://svn.chromium.org/blink/trunk@165784 bbb929c8-8fbe-4397-9dbb-9b2b20218538