Bug 1623004 - Modify content-length header with change in custom post data r=Honza

Differential Revision: https://phabricator.services.mozilla.com/D69574

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

devtools/client/netmonitor/src/components/CustomRequestPanel.js

@@ -125,10 +125,26 @@ class CustomRequestPanel extends Component {
             : { method: val.trim() };
         break;
       case "custom-postdata-value":
+        // Update "content-length" header value to reflect change
+        // in post data field.
+        const { requestHeaders } = request;
+        const newHeaders = requestHeaders.headers.map(header => {
+          if (header.name.toLowerCase() == "content-length") {
+            return {
+              name: header.name,
+              value: val.length,
+            };
+          }
+          return header;
+        });
+
         data = {
           requestPostData: {
             postData: { text: val },
           },
+          requestHeaders: {
+            headers: newHeaders,
+          },
         };
         break;
       case "custom-query-value":