Enable stable features by default

Currently the embedder is required to do this, but it doesn't happen under some
tests (eg. those which use MockRenderThread). As it's not clear when an
embedder should be able to disable stable features, that functionality is also
removed.

Review URL: https://codereview.chromium.org/159753006

git-svn-id: svn://svn.chromium.org/blink/trunk@166996 bbb929c8-8fbe-4397-9dbb-9b2b20218538