Ensure SimpleURLLoader callbacks are set prior to request being started

CL makes sure that SetOnResponseStartedCallback and SetOnRedirectCallback
can only be set prior to the request gets started.

This is a follow up on a request made by mmenke@ in [1].

[1] https://crrev.com/c/1142350/14/services/network/public/cpp/simple_url_loader.cc#1226

TBR=pfeldman@chromium.org,peter@chromium.org

BUG=773295

Change-Id: If937313c21df5139f1c6e21f982b228513501962
Reviewed-on: https://chromium-review.googlesource.com/c/1317949
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605417}

chrome/browser/devtools/devtools_ui_bindings.cc

@@ -363,9 +363,9 @@ class DevToolsUIBindings::NetworkResourceLoader
         bindings_(bindings),
         loader_(std::move(loader)),
         callback_(callback) {
-    loader_->DownloadAsStream(url_loader_factory, this);
     loader_->SetOnResponseStartedCallback(base::BindOnce(
         &NetworkResourceLoader::OnResponseStarted, base::Unretained(this)));
+    loader_->DownloadAsStream(url_loader_factory, this);
   }
 
  private:

content/shell/browser/shell_devtools_bindings.cc

@@ -77,9 +77,9 @@ class ShellDevToolsBindings::NetworkResourceLoader
         request_id_(request_id),
         bindings_(bindings),
         loader_(std::move(loader)) {
-    loader_->DownloadAsStream(url_loader_factory, this);
     loader_->SetOnResponseStartedCallback(base::BindOnce(
         &NetworkResourceLoader::OnResponseStarted, base::Unretained(this)));
+    loader_->DownloadAsStream(url_loader_factory, this);
   }
 
  private:

services/network/public/cpp/simple_url_loader.cc

@@ -1226,11 +1226,18 @@ void SimpleURLLoaderImpl::DownloadAsStream(
 
 void SimpleURLLoaderImpl::SetOnRedirectCallback(
     const OnRedirectCallback& on_redirect_callback) {
+  // Check if a request has not yet been started.
+  DCHECK(!body_handler_);
+
   on_redirect_callback_.push_back(on_redirect_callback);
+  DCHECK(on_redirect_callback);
 }
 
 void SimpleURLLoaderImpl::SetOnResponseStartedCallback(
     OnResponseStartedCallback on_response_started_callback) {
+  // Check if a request has not yet been started.
+  DCHECK(!body_handler_);
+
   on_response_started_callback_ = std::move(on_response_started_callback);
   DCHECK(on_response_started_callback_);
 }