Add a virtual test suite for exposed "stable" features

This change removes enabling all the runtime enabled features in
TestInterfaces. This code gone, we can use the new stable flag
to add a test suite for "stable" features only (yay!).

BUG=234853

Review URL: https://chromiumcodereview.appspot.com/22897010

git-svn-id: svn://svn.chromium.org/blink/trunk@156570 bbb929c8-8fbe-4397-9dbb-9b2b20218538