Add a Runtime Flag for CSS Shapes

With the split between CSS Shapes and CSS Exclusions, CSS Shapes should
have its own runtime flag. This patch adds the runtime flag and the
connections to make it toggle-able in tests.

BUG=248304

Review URL: https://chromiumcodereview.appspot.com/23227002

git-svn-id: svn://svn.chromium.org/blink/trunk@156171 bbb929c8-8fbe-4397-9dbb-9b2b20218538