derp/derphttp: don't link websockets other than on GOOS=js

Or unless the new "ts_debug_websockets" build tag is set.

Updates #1278

Change-Id: Ic4c4f81c1924250efd025b055585faec37a5491d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlhttp/client_js.go

@@ -12,6 +12,7 @@ import (
 
 	"github.com/coder/websocket"
 	"tailscale.com/control/controlbase"
+	"tailscale.com/control/controlhttp/controlhttpcommon"
 	"tailscale.com/net/wsconn"
 )
 
@@ -42,11 +43,11 @@ func (d *Dialer) Dial(ctx context.Context) (*ClientConn, error) {
 		// Can't set HTTP headers on the websocket request, so we have to to send
 		// the handshake via an HTTP header.
 		RawQuery: url.Values{
-			handshakeHeaderName: []string{base64.StdEncoding.EncodeToString(init)},
+			controlhttpcommon.HandshakeHeaderName: []string{base64.StdEncoding.EncodeToString(init)},
 		}.Encode(),
 	}
 	wsConn, _, err := websocket.Dial(ctx, wsURL.String(), &websocket.DialOptions{
-		Subprotocols: []string{upgradeHeaderValue},
+		Subprotocols: []string{controlhttpcommon.UpgradeHeaderValue},
 	})
 	if err != nil {
 		return nil, err