[wdspec] Improvements for WebDriver BiDi Python client.

The "session.new" command currently doesn't work because the
transport is started after sending the command. This needed
to be flipped.

Also it is helpful to have the event loop of the current
transport available as property on the BiDi session directly.

Differential Revision: https://phabricator.services.mozilla.com/D144979

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1719505
gecko-commit: 7ac82aff1852eaf89acc99714a7359596b093d2a
gecko-reviewers: webdriver-reviewers, jgraham

tools/webdriver/webdriver/bidi/client.py

@@ -91,6 +91,13 @@ def __init__(self,
         self.session = modules.Session(self)
         self.browsing_context = modules.BrowsingContext(self)
 
+    @property
+    def event_loop(self):
+        if self.transport:
+            return self.transport.loop
+
+        return None
+
     @classmethod
     def from_http(cls,
                   session_id: str,
@@ -109,7 +116,7 @@ def from_http(cls,
     @classmethod
     def bidi_only(cls,
                   websocket_url: str,
-                  requested_capabilities: Optional[Mapping[str, Any]]) -> "BidiSession":
+                  requested_capabilities: Optional[Mapping[str, Any]] = None) -> "BidiSession":
         """Create a BiDi session where there is no existing HTTP session
 
         :param webdocket_url: URL to the WebSocket server listening for BiDi connections
@@ -129,13 +136,13 @@ async def start(self,
 
         if loop is None:
             loop = get_running_loop()
+
         self.transport = Transport(self.websocket_url, self.on_message, loop=loop)
+        await self.transport.start()
 
         if self.session_id is None:
             self.session_id, self.capabilities = await self.session.new(
-                self.requested_capabilities)
-
-        await self.transport.start()
+                capabilities=self.requested_capabilities)
 
     async def send_command(
         self,