Add a field trial to delay all resource requests by thread hops.

chrome/browser/renderer_host/thread_hop_resource_throttle.h

Index: chrome/browser/renderer_host/thread_hop_resource_throttle.h
diff --git a/chrome/browser/renderer_host/thread_hop_resource_throttle.h b/chrome/browser/renderer_host/thread_hop_resource_throttle.h
new file mode 100644
index 0000000000000000000000000000000000000000..1de718c305672a9e8304164ba70f0b64c7182287
--- /dev/null
+++ b/chrome/browser/renderer_host/thread_hop_resource_throttle.h
@@ -0,0 +1,43 @@
+// Copyright 2015 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.
+
+#ifndef CHROME_BROWSER_RENDERER_HOST_THREAD_HOP_RESOURCE_THROTTLE_H_
+#define CHROME_BROWSER_RENDERER_HOST_THREAD_HOP_RESOURCE_THROTTLE_H_
+
+#include "base/macros.h"
+#include "base/memory/weak_ptr.h"
+#include "content/public/browser/resource_throttle.h"
+
+namespace base {
+class TimeTicks;
+}
+
+// A ResourceThrottle which defers request start, redirect, and response
+// completion on an IO to UI to IO round-trip. This throttle will be added on a
+// field trial to determine the cost of making some resource-related decisions
+// on the UI thread. See https://crbug.com/524228.
+class ThreadHopResourceThrottle : public content::ResourceThrottle {
+ public:
+  ThreadHopResourceThrottle();
+  ~ThreadHopResourceThrottle() override;
+
+  static bool IsEnabled();
+
+  // content::ResourceThrottle implementation:

[Alexei Svitkine (slow), 2015/10/02 16:42:54]

Nit: I think the new suggested content for this type of comment is:

// content::ResourceThrottle:

[davidben, 2015/10/02 17:39:39]

Done.

+  void WillStartRequest(bool* defer) override;
+  void WillRedirectRequest(const net::RedirectInfo& redirect_info,
+                           bool* defer) override;
+  void WillProcessResponse(bool* defer) override;
+  const char* GetNameForLogging() const override;
+
+ private:
+  void ResumeAfterThreadHop();
+  void Resume(const base::TimeTicks& time);
+
+  base::WeakPtrFactory<ThreadHopResourceThrottle> weak_factory_;
+
+  DISALLOW_COPY_AND_ASSIGN(ThreadHopResourceThrottle);
+};
+
+#endif  // CHROME_BROWSER_RENDERER_HOST_THREAD_HOP_RESOURCE_THROTTLE_H_