perf(net): Avoid parsing headers twice (#8618)

Author: avelad

lib/net/http_fetch_plugin.js

@@ -113,6 +113,7 @@ shaka.net.HttpFetchPlugin = class {
     let arrayBuffer = new ArrayBuffer(0);
     let loaded = 0;
     let lastLoaded = 0;
+    let headers = {};
 
     // Last time stamp when we got a progress event.
     let lastTime = Date.now();
@@ -124,8 +125,9 @@ shaka.net.HttpFetchPlugin = class {
       response = await fetch(uri, init);
       // At this point in the process, we have the headers of the response, but
       // not the body yet.
-      headersReceived(shaka.net.HttpFetchPlugin.headersToGenericObject_(
-          response.headers));
+      headers =
+          shaka.net.HttpFetchPlugin.headersToGenericObject_(response.headers);
+      headersReceived(headers);
 
       // In new versions of Chromium, HEAD requests now have a response body
       // that is null.
@@ -237,9 +239,6 @@ shaka.net.HttpFetchPlugin = class {
       }
     }
 
-    const headers = shaka.net.HttpFetchPlugin.headersToGenericObject_(
-        response.headers);
-
     return shaka.net.HttpPluginUtils.makeResponse(headers, arrayBuffer,
         response.status, uri, response.url, request, requestType);
   }