Add enabling/disabling of NetworkInformation API via WebRuntimeFeatures.  This lets us restrict the API to the platforms that are fully implemented while we develop the others.

See https://codereview.chromium.org/424823002/ for the Chromium code that uses this.

BUG=368358

Review URL: https://codereview.chromium.org/422933003

git-svn-id: svn://svn.chromium.org/blink/trunk@179147 bbb929c8-8fbe-4397-9dbb-9b2b20218538