With --isolate-extensions, forking (via OpenURL) for extensions is not needed.

chrome/renderer/chrome_content_renderer_client.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/renderer/chrome_content_renderer_client.h"
 
 #include <memory>
 #include <utility>
 
 #include "base/bind.h"
@@ skipping 1121 matching lines @@
   // process if it is an Instant url, or to another process if not.  Conversely,
   // fork if this is a non-Instant process navigating to an Instant url, so that
   // such navigations can also be bucketed into an Instant renderer.
   if (base::CommandLine::ForCurrentProcess()->HasSwitch(
           switches::kInstantProcess) ||
       SearchBouncer::GetInstance()->ShouldFork(url)) {
     *send_referrer = true;
     return true;
   }
 
-  // TODO(lukasza): https://crbug.com/650694: For now, we skip the rest for POST
-  // submissions.  This is because 1) in M54 there are some remaining issues
-  // with POST in OpenURL path (e.g. https://crbug.com/648648) and 2) OpenURL
-  // path regresses (blocks) navigations that result in downloads
-  // (https://crbug.com/646261).  In the long-term we should avoid forking for
-  // extensions (not hosted apps though) altogether and rely on transfers logic
-  // instead.
-  if (http_method != "GET")
-    return false;
-
   // If |url| matches one of the prerendered URLs, stop this navigation and try
   // to swap in the prerendered page on the browser process. If the prerendered
   // page no longer exists by the time the OpenURL IPC is handled, a normal
   // navigation is attempted.
   if (prerender_dispatcher_.get() &&
       prerender_dispatcher_->IsPrerenderURL(url)) {
     *send_referrer = true;
     return true;
   }
 
@@ skipping 392 matching lines @@
 
   RecordYouTubeRewriteUMA(result);
   return corrected_url.ReplaceComponents(r);
 }
 
 std::unique_ptr<base::TaskScheduler::InitParams>
 ChromeContentRendererClient::GetTaskSchedulerInitParams() {
   return task_scheduler_util::
       GetRendererTaskSchedulerInitParamsFromCommandLine();
 }