safaridriver: add support for an extension command to open the debugger.

Upon executing this command, Safari will open the Web Inspector and
pause at a `debugger;` statement.

Author: burg

py/selenium/webdriver/safari/remote_connection.py

@@ -24,3 +24,4 @@ def __init__(self, remote_server_addr, keep_alive=True):
 
         self._commands["GET_PERMISSIONS"] = ('GET', '/session/$sessionId/apple/permissions')
         self._commands["SET_PERMISSIONS"] = ('POST', '/session/$sessionId/apple/permissions')
+        self._commands["ATTACH_DEBUGGER"] = ('POST', '/session/$sessionId/apple/attach_debugger')

py/selenium/webdriver/safari/webdriver.py

@@ -99,3 +99,8 @@ def get_permission(self, permission):
             return None
 
         return value
+
+    # First available in Safari 11.1 and Safari Technology Preview 42.
+    def debug(self):
+        self.execute("ATTACH_DEBUGGER")
+        self.execute_script("debugger;")