Remove -webkit-svg-shadow

This patch removes the non-standard -webkit-svg-shadow property per the intent to deprecate thread on blink-dev: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/Rsq5BecyyNU

BUG=262857

Review URL: https://chromiumcodereview.appspot.com/20237002

git-svn-id: svn://svn.chromium.org/blink/trunk@154982 bbb929c8-8fbe-4397-9dbb-9b2b20218538