Custom select - add impl_url for Firefox and Safari (#28727)

Added implementation URLs for Firefox and Safari.

Author: ramiy

html/elements/selectedcontent.json

@@ -15,14 +15,16 @@
             "chrome_android": "mirror",
             "edge": "mirror",
             "firefox": {
-              "version_added": false
+              "version_added": false,
+              "impl_url": "https://bugzil.la/1974276"
             },
             "firefox_android": "mirror",
             "oculus": "mirror",
             "opera": "mirror",
             "opera_android": "mirror",
             "safari": {
-              "version_added": false
+              "version_added": false,
+              "impl_url": "https://webkit.org/b/301929"
             },
             "safari_ios": "mirror",
             "samsunginternet_android": "mirror",