Add file:// as possible origin

Summary:
For Flipper Desktop, Electron based, the origin maybe file:// as shown in tests:

```
Refused socket connection from cross domain request, origin: file://, host: localhost:52342.

Expected origins: http://localhost:52342 or http://[::1]:52342 or http://::1:52342 or http://localhost:3000.
Expected hosts: localhost:52342 or [::1]:52342 or ::1:52342 or localhost:3000
```

This change effectively adds `file://` as a valid origin.

Reviewed By: passy

Differential Revision: D37212809

fbshipit-source-id: 311a93bb8872587d15c6be4329238af6b0f6988d

desktop/flipper-server-core/src/server/startServer.tsx

@@ -160,7 +160,9 @@ function addWebsocket(server: http.Server, config: Config) {
     localhostIPV6NoBrackets,
     localhostIPV4Electron,
   ];
-  const possibleOrigins = possibleHosts.map((host) => `http://${host}`);
+  const possibleOrigins = possibleHosts
+    .map((host) => `http://${host}`)
+    .concat(['file://']);
 
   const verifyClient: VerifyClientCallbackSync = ({origin, req}) => {
     const noOriginHeader = origin === undefined;