Bug 246784

Steps to reproduce:

1. Go to https://wpt.fyi/results/input-events/input-events-typing.html?label=experimental&label=master&aligned
2. Select Show Details on the top right

Expected:
FAIL or PASS

Actual:
Error message from Webdriver.
webdriver.error.InvalidArgumentException: invalid argument (400): Encountered key input source with invalid 'value' in payload



```
ERROR message: Encountered key input source with invalid 'value' in payload: {
    actions =     (
                {
            type = keyDown;
            value = "\U2318";
        },
                {
            type = keyDown;
            value = z;
        },
                {
            type = keyUp;
            value = z;
        },
                {
            type = keyUp;
            value = "\U2318";
        }
    );
    id = 3;
    type = key;
}
Traceback (most recent call last):
  File "/Users/runner/work/1/s/tools/wptrunner/wptrunner/executors/executorwebdriver.py", line 431, in run_func
    self.result = True, self.func(self.protocol, self.url, self.timeout)
  File "/Users/runner/work/1/s/tools/wptrunner/wptrunner/executors/executorwebdriver.py", line 537, in do_testharness
    done, rv = handler(result)
  File "/Users/runner/work/1/s/tools/wptrunner/wptrunner/executors/base.py", line 724, in __call__
    return callback(url, payload)
  File "/Users/runner/work/1/s/tools/wptrunner/wptrunner/executors/base.py", line 740, in process_action
    result = action_handler(payload)
  File "/Users/runner/work/1/s/tools/wptrunner/wptrunner/executors/actions.py", line 112, in __call__
    self.protocol.action_sequence.send_actions({"actions": actions})
  File "/Users/runner/work/1/s/tools/wptrunner/wptrunner/executors/executorwebdriver.py", line 249, in send_actions
    self.webdriver.actions.perform(actions['actions'])
  File "/Users/runner/work/1/s/tools/webdriver/webdriver/client.py", line 22, in inner
    return func(self, *args, **kwargs)
  File "/Users/runner/work/1/s/tools/webdriver/webdriver/client.py", line 283, in perform
    actions = self.session.send_session_command("POST", "actions", body)
  File "/Users/runner/work/1/s/tools/webdriver/webdriver/client.py", line 661, in send_session_command
    return self.send_command(method, url, body, timeout)
  File "/Users/runner/work/1/s/tools/webdriver/webdriver/client.py", line 625, in send_command
    raise err
webdriver.error.InvalidArgumentException: invalid argument (400): Encountered key input source with invalid 'value' in payload: {
    actions =     (
                {
            type = keyDown;
            value = "\U2318";
        },
                {
            type = keyDown;
            value = z;
        },
                {
            type = keyUp;
            value = z;
        },
                {
            type = keyUp;
            value = "\U2318";
        }
    );
    id = 3;
    type = key;
}
```