Fixed SeleniumTestCase.set_emulated_media() for Selenium Grid.

When using Selenium Grid via the `--selenium-hub` option, we cannot use
the `.execute_cdp_cmd()` method which doesn't exist on the remote
driver.

django/test/selenium.py

@@ -193,13 +193,25 @@ def dark(self):
             finally:
                 self.selenium.execute_script("localStorage.removeItem('theme');")
 
-    def set_emulated_media(self, features, media=""):
+    def set_emulated_media(self, *, media=None, features=None):
         if self.browser not in {"chrome", "edge"}:
             self.skipTest("Emulated media controls are only supported on Chrome/Edge.")
+
         # Chrome Dev Tools Protocol Emulation.setEmulatedMedia
         # https://chromedevtools.github.io/devtools-protocol/1-3/Emulation/#method-setEmulatedMedia
-        self.selenium.execute_cdp_cmd(
-            "Emulation.setEmulatedMedia", {"media": media, "features": features}
+
+        # Not using .execute_cdp_cmd() as it isn't supported by the remote web driver
+        # when using Selenium Grid.
+
+        params = {}
+        if media is not None:
+            params["media"] = media
+        if features is not None:
+            params["features"] = features
+
+        self.selenium.execute(
+            driver_command="executeCdpCommand",
+            params={"cmd": "Emulation.setEmulatedMedia", "params": params},
         )
 
     @contextmanager