Remove explicit `document` receiver from add/removeEventListener calls

This allows ActionCable to be used in a web worker, where the `document`
global is undefined. Previously, attempting to use ActionCable inside a
web worker would result in this exception after you try to open a
connection:
```
ReferenceError: document is not defined
```

The visibilitychange event won't ever get triggered in a worker, so
adding the listener is effectively a no-op there. But the listener is
mainly a convenience, rather than a critical piece of the javascript
interface, so using ActionCable in a worker will still work. (And you
could listen for visibilitychange yourself in a window script, then tell
the worker to reconnect if you still want that behavior.)

actioncable/app/assets/javascripts/action_cable.js

@@ -49,7 +49,7 @@
         this.startedAt = now();
         delete this.stoppedAt;
         this.startPolling();
-        document.addEventListener("visibilitychange", this.visibilityDidChange);
+        addEventListener("visibilitychange", this.visibilityDidChange);
         logger.log("ConnectionMonitor started. pollInterval = " + this.getPollInterval() + " ms");
       }
     };
@@ -57,7 +57,7 @@
       if (this.isRunning()) {
         this.stoppedAt = now();
         this.stopPolling();
-        document.removeEventListener("visibilitychange", this.visibilityDidChange);
+        removeEventListener("visibilitychange", this.visibilityDidChange);
         logger.log("ConnectionMonitor stopped");
       }
     };

actioncable/app/javascript/action_cable/connection_monitor.js

@@ -21,7 +21,7 @@ class ConnectionMonitor {
       this.startedAt = now()
       delete this.stoppedAt
       this.startPolling()
-      document.addEventListener("visibilitychange", this.visibilityDidChange)
+      addEventListener("visibilitychange", this.visibilityDidChange)
       logger.log(`ConnectionMonitor started. pollInterval = ${this.getPollInterval()} ms`)
     }
   }
@@ -30,7 +30,7 @@ class ConnectionMonitor {
     if (this.isRunning()) {
       this.stoppedAt = now()
       this.stopPolling()
-      document.removeEventListener("visibilitychange", this.visibilityDidChange)
+      removeEventListener("visibilitychange", this.visibilityDidChange)
       logger.log("ConnectionMonitor stopped")
     }
   }