Introduce WebSocketHandle and WebSocketHandleClient.

Introduce WebSocketHandle and WebSocketHandleClient in public/platform.
They are interface classes for the new WebSocket implementation.

This CL does not change any behavior.

BUG=275459
R=tyoshino, abarth

Review URL: https://chromiumcodereview.appspot.com/23684002

git-svn-id: svn://svn.chromium.org/blink/trunk@156834 bbb929c8-8fbe-4397-9dbb-9b2b20218538