servodriver: Use the wpt-prefs.json file when running servoshell (#39605)

These preferences are necessary in order for tests to run properly in
servodriver.

Testing: This causes more tests to pass when WPT is run with `--product
servodriver`

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

tests/wpt/meta/MANIFEST.json

@@ -529727,7 +529727,7 @@
         []
        ],
        "servodriver.py": [
-        "c25aa9b3ecdcfe20c6ff0e8f98973d69c86d5209",
+        "a4bf5e7cd75bbcabce532930ee97bfd506e06fc4",
         []
        ],
        "webkit.py": [

tests/wpt/tests/tools/wptrunner/wptrunner/browsers/servodriver.py

@@ -113,9 +113,13 @@ class ServoWebDriverBrowser(WebDriverBrowser):
         if headless:
             args += ["--headless"]
 
+        # Add the shared `wpt-prefs.json` file to the list of arguments.
+        args += ["--prefs-file", self.find_wpt_prefs(logger)]
+
         super().__init__(logger, binary=binary, webdriver_binary=binary,
                          webdriver_args=args, host=webdriver_host, env=env,
                          supports_pac=False, **kwargs)
+
         self.hosts_path = hosts_path
 
     def make_command(self):
@@ -124,3 +128,17 @@ class ServoWebDriverBrowser(WebDriverBrowser):
     def cleanup(self):
         super().cleanup()
         os.remove(self.hosts_path)
+
+    def find_wpt_prefs(self, logger):
+        default_path = os.path.join("resources", "wpt-prefs.json")
+        # The cwd is the servo repo for `./mach test-wpt`, but on WPT runners
+        # it is the WPT repo. The nightly tar is extracted inside the Python
+        # virtual environment within the repo. This means that on WPT runners,
+        # the cwd has the `_venv3/servo` directory inside which we find the
+        # binary and the 'resources' directory.
+        for dir in [".", "./_venv3/servo"]:
+            candidate = os.path.abspath(os.path.join(dir, default_path))
+            if os.path.isfile(candidate):
+                return candidate
+        logger.error("Unable to find wpt-prefs.json")
+        return default_path