Use webkit-margin-end for details element to get same margin for both ltr and rtl

RenderDetailMarker needs both left and right margin to get rendered with same margin. Updated with webkit-margin-end to get the same margin result for both ltr and rtl

R=eseidel, leviw, tkent
BUG=335075
TEST=Existing details-writing-mode covers test for showing text in RTL box, with the patch rtl box now shows margin which is currently missing.

Review URL: https://codereview.chromium.org/306923003

git-svn-id: svn://svn.chromium.org/blink/trunk@176016 bbb929c8-8fbe-4397-9dbb-9b2b20218538