[wptrunner] Fix missing WebDriverCrashtestExecutor.is_alive() method

The `TestDriverExecutorMixin` introduced for #13183 requires an
`is_alive()` executor method, but we forgot to add one for
`WebDriverCrashtestExecutor` [0]:

```
2025-03-17 05:05:54.246 INFO Starting runner
  ▶ ERROR [expected PASS] external/wpt/html/semantics/forms/the-select-element/customizable-select/nested-select-crash.html
  └   → Traceback (most recent call last):
  File "/Volumes/Work/s/w/ith4ri8l3q/external/wpt/tools/wptrunner/wptrunner/executors/executorwebdriver.py", line 831, in run_func
    self.result = True, self.func(self.protocol, self.url, self.timeout)
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Volumes/Work/s/w/ith4ri8l3q/external/wpt/tools/wptrunner/wptrunner/executors/executorwebdriver.py", line 1239, in do_crashtest
    self.run_testdriver(protocol, url, timeout)
  File "/Volumes/Work/s/w/ith4ri8l3q/external/wpt/tools/wptrunner/wptrunner/executors/executorwebdriver.py", line 938, in run_testdriver
    is_alive = self.is_alive()
               ^^^^^^^^^^^^^
AttributeError: 'WebDriverCrashtestExecutor' object has no attribute 'is_alive'
```

Fixed by just inlining `protocol.is_alive()`, which is the same for all
executor types.

[0]: https://chromium-swarm.appspot.com/task?id=6fbcb3db3c2e6410

Author: jonathan-j-lee

Diff for: tools/wptrunner/wptrunner/executors/executorwebdriver.py

@@ -859,8 +859,6 @@ def run_func(self):
             self.result_flag.set()
 
 
-# TODO(web-platform-tests/wpt#13183): Add testdriver support to the other
-# executors.
 class TestDriverExecutorMixin:
     def __init__(self, script_resume: str):
         self.script_resume = script_resume
@@ -935,7 +933,7 @@ async def process_bidi_event(method, params):
             # https://github.com/w3c/webdriver/issues/1308
             if not isinstance(test_driver_message, list) or len(test_driver_message) != 3:
                 try:
-                    is_alive = self.is_alive()
+                    is_alive = protocol.is_alive()
                 except webdriver_error.WebDriverException:
                     is_alive = False
                 if not is_alive:
@@ -990,9 +988,6 @@ def __init__(self, logger, browser, server_config, timeout_multiplier=1,
         self.close_after_done = close_after_done
         self.cleanup_after_test = cleanup_after_test
 
-    def is_alive(self):
-        return self.protocol.is_alive()
-
     def on_environment_change(self, new_environment):
         if new_environment["protocol"] != self.last_environment["protocol"]:
             self.protocol.testharness.load_runner(new_environment["protocol"])
@@ -1090,9 +1085,6 @@ def __init__(self, logger, browser, server_config, timeout_multiplier=1,
     def reset(self):
         self.implementation.reset()
 
-    def is_alive(self):
-        return self.protocol.is_alive()
-
     def do_test(self, test):
         width_offset, height_offset = self.protocol.webdriver.execute_script(
             """return [window.outerWidth - window.innerWidth,