Move ExperimentalWebSocket flag to RuntimeEnabledFeatures, Part 1

Move ExperimentalWebSocket flag from Settings to RuntimeEnabledFeatures to
enable it in LayoutTests.
Note: In this CL, the flag is not enabled in LayoutTests.
It is OK, This is just a preparation.

BUG=275459

Review URL: https://codereview.chromium.org/145553011

git-svn-id: svn://svn.chromium.org/blink/trunk@165827 bbb929c8-8fbe-4397-9dbb-9b2b20218538