Remove no-longer-necessary platform/default stub for WebThemeEngine.h.

Apparently I missed this when removing the others in r167145.

R=jamesr@chromium.org
BUG=327471

Review URL: https://codereview.chromium.org/166273025

git-svn-id: svn://svn.chromium.org/blink/trunk@167626 bbb929c8-8fbe-4397-9dbb-9b2b20218538