[facebook] remove hardcoded chrome user-agent

closes ytdl-org#18974
closes ytdl-org#25411
closes ytdl-org#26958
closes ytdl-org#27329

youtube_dl/extractor/facebook.py

@@ -54,8 +54,6 @@ class FacebookIE(InfoExtractor):
     _NETRC_MACHINE = 'facebook'
     IE_NAME = 'facebook'
 
-    _CHROME_USER_AGENT = 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/48.0.2564.97 Safari/537.36'
-
     _VIDEO_PAGE_TEMPLATE = 'https://www.facebook.com/video/video.php?v=%s'
     _VIDEO_PAGE_TAHOE_TEMPLATE = 'https://www.facebook.com/video/tahoe/async/%s/?chain=true&isvideo=true&payloadtype=primary'
 
@@ -306,9 +304,7 @@ def _real_initialize(self):
         self._login()
 
     def _extract_from_url(self, url, video_id, fatal_if_no_video=True):
-        req = sanitized_Request(url)
-        req.add_header('User-Agent', self._CHROME_USER_AGENT)
-        webpage = self._download_webpage(req, video_id)
+        webpage = self._download_webpage(url, video_id)
 
         video_data = None