Hard codes ScrollbarThemeGtkOrAura::scrollbarThickness to 15 in layout test mode

As part of 333499 the width of scrollbars under windows is going to
honor what the OS says (as it was before aura). For tests the size
needs to be hardcoded (we had been doing this in ScrollbarThemeWin.cpp).

BUG=333499 332937
TEST=none
R=dpranke@chromium.org, jamesr@chromium.org

Review URL: https://codereview.chromium.org/144173002

git-svn-id: svn://svn.chromium.org/blink/trunk@165497 bbb929c8-8fbe-4397-9dbb-9b2b20218538