Make NewWebSocketChannelImpl a ContextLifeCycleObserver subclass.

NewWebSocketChannelImpl should have a ScriptExecutionContext.
To prevent accessing the context after it is destroyed, we make NewWebSocketChannelImpl a ContextLifeCycleObserver subclass.
Currently NewWebSocketChannelImpl is destroyed before the context is destroyed and hence contextDestroyed shoule never be called.

BUG=275459
R=tyoshino, abarth

Review URL: https://chromiumcodereview.appspot.com/23523014

git-svn-id: svn://svn.chromium.org/blink/trunk@157134 bbb929c8-8fbe-4397-9dbb-9b2b20218538