[youtube] Add `mobile_web` client (#557)

Authored by: colethedj

README.md

@@ -1354,7 +1354,7 @@ Some extractors accept additional arguments which can be passed using `--extract
 The following extractors use this feature:
 * **youtube**
     * `skip`: `hls` or `dash` (or both) to skip download of the respective manifests
-    * `player_client`: Clients to extract video data from - one or more of `web`, `android`, `ios`, `web_music`, `android_music`, `ios_music` or `all`. By default, `android,web` is used. If the URL is from `music.youtube.com`, `android,web,android_music,web_music` is used
+    * `player_client`: Clients to extract video data from - one or more of `web`, `android`, `ios`, `mobile_web`, `web_music`, `android_music`, `ios_music` or `all`. By default, `android,web` is used. If the URL is from `music.youtube.com`, `android,web,android_music,web_music` is used
     * `player_skip`: `configs` - skip any requests for client configs and use defaults
     * `comment_sort`: `top` or `new` (default) - choose comment sorting mode (on YouTube's side).
     * `max_comments`: maximum amount of comments to download (default all).

yt_dlp/extractor/youtube.py

@@ -439,7 +439,21 @@ class YoutubeBaseInfoExtractor(InfoExtractor):
                 }
             },
             'INNERTUBE_CONTEXT_CLIENT_NAME': 66
-        }
+        },
+        'MWEB': {
+            'INNERTUBE_API_VERSION': 'v1',
+            'INNERTUBE_CLIENT_NAME': 'MWEB',
+            'INNERTUBE_CLIENT_VERSION': '2.20210721.07.00',
+            'INNERTUBE_API_KEY': 'AIzaSyDCU8hByM-4DrUqRUYnGn-3llEO78bcxq8',
+            'INNERTUBE_CONTEXT': {
+                'client': {
+                    'clientName': 'MWEB',
+                    'clientVersion': '2.20210721.07.00',
+                    'hl': 'en',
+                }
+            },
+            'INNERTUBE_CONTEXT_CLIENT_NAME': 2
+        },
     }
 
     _YT_DEFAULT_INNERTUBE_HOSTS = {
@@ -463,6 +477,7 @@ class YoutubeBaseInfoExtractor(InfoExtractor):
         'web_music': 'WEB_REMIX',
         '_web_embedded': 'WEB_EMBEDDED_PLAYER',
         '_web_agegate': 'TVHTML5',
+        'mobile_web': 'MWEB',
     }
 
     def _get_default_ytcfg(self, client='WEB'):