Bug 1546884 - fix third-party channel checks; r=dragana

`mThirdPartyClassificationFlags` can only check for third-party tracking channel, it should be checked by calling `IsThirdPartyWindowOrChannel`.

Differential Revision: https://phabricator.services.mozilla.com/D28788

--HG--
extra : moz-landing-system : lando

netwerk/protocol/http/nsHttpChannel.cpp

@@ -1333,7 +1333,8 @@ HttpTrafficCategory nsHttpChannel::CreateTrafficCategory() {
     }
   }
 
-  bool isThirdParty = !!mThirdPartyClassificationFlags;
+  bool isThirdParty =
+      nsContentUtils::IsThirdPartyWindowOrChannel(nullptr, this, mURI);
   HttpTrafficAnalyzer::TrackingClassification tc;
   {
     uint32_t flags = isThirdParty ? mThirdPartyClassificationFlags