Bug 1871963: Implement zstd content-encoding support r=necko-reviewers,valentin,devtools-reviewers

Differential Revision: https://phabricator.services.mozilla.com/D205109

netwerk/protocol/http/HttpBaseChannel.cpp

@@ -1539,6 +1539,8 @@ HttpBaseChannel::DoApplyContentConversions(nsIStreamListener* aNextListener,
           mode = 2;
         } else if (from.EqualsLiteral("br")) {
           mode = 3;
+        } else if (from.EqualsLiteral("zstd")) {
+          mode = 4;
         }
         Telemetry::Accumulate(Telemetry::HTTP_CONTENT_ENCODING, mode);
       }
@@ -1643,6 +1645,14 @@ HttpBaseChannel::nsContentEncodings::GetNext(nsACString& aNextEncoding) {
     }
   }
 
+  if (!haveType) {
+    encoding.BeginReading(start);
+    if (CaseInsensitiveFindInReadable("zstd"_ns, start, end)) {
+      aNextEncoding.AssignLiteral(APPLICATION_ZSTD);
+      haveType = true;
+    }
+  }
+
   // Prepare to fetch the next encoding
   mCurEnd = mCurStart;
   mReady = false;