Remove unused WebRuntimeFeatures::enableStableFeatures

As of chromium r254438 this method is never called.

[1] https://src.chromium.org/viewvc/chrome?view=rev&revision=254438

Review URL: https://codereview.chromium.org/205733003

git-svn-id: svn://svn.chromium.org/blink/trunk@169664 bbb929c8-8fbe-4397-9dbb-9b2b20218538