MimeSniffingURLLoader: Always forward empty response's body.

This is similar to:
 * https://chromium-review.googlesource.com/c/chromium/src/+/1323109
 * https://chromium-review.googlesource.com/c/chromium/src/+/1329163
 * https://chromium-review.googlesource.com/c/chromium/src/+/1323070
 * https://chromium-review.googlesource.com/c/chromium/src/+/1323092

On request success, it makes the MimeSniffingURLLoader to always send a
response's body datapipe, even when it is empty.

It also adds a DCHECK to ensure the |source_url_loader| is also behaving
the same way.

Bug: 831155,826868
Change-Id: I468a24c31142cb0a8d646cd09928f7b03680d779
Reviewed-on: https://chromium-review.googlesource.com/c/1350874Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612152}

content/common/mime_sniffing_throttle_unittest.cc

@@ -299,6 +299,33 @@ TEST_F(MimeSniffingThrottleTest, NoBody) {
   EXPECT_TRUE(delegate->is_intercepted());
 
   // Call OnComplete() without sending body.
+  delegate->source_loader_client()->OnComplete(
+      network::URLLoaderCompletionStatus(net::ERR_FAILED));
+  delegate->destination_loader_client()->RunUntilComplete();
+
+  // The mime type should be updated to the default mime type ("text/plain").
+  EXPECT_TRUE(delegate->destination_loader_client()->has_received_response());
+  EXPECT_EQ("text/plain",
+            delegate->destination_loader_client()->response_head().mime_type);
+}
+
+TEST_F(MimeSniffingThrottleTest, EmptyBody) {
+  auto throttle = std::make_unique<MimeSniffingThrottle>();
+  auto delegate = std::make_unique<MockDelegate>();
+  throttle->set_delegate(delegate.get());
+
+  GURL response_url("https://example.com");
+  network::ResourceResponseHead response_head;
+  bool defer = false;
+  throttle->WillProcessResponse(response_url, &response_head, &defer);
+  EXPECT_TRUE(defer);
+  EXPECT_TRUE(delegate->is_intercepted());
+
+  mojo::DataPipe pipe;
+  delegate->source_loader_client()->OnStartLoadingResponseBody(
+      std::move(pipe.consumer_handle));
+  pipe.producer_handle.reset();  // The pipe is empty.
+
   delegate->source_loader_client()->OnComplete(
       network::URLLoaderCompletionStatus());
   delegate->destination_loader_client()->RunUntilComplete();

content/common/mime_sniffing_url_loader.cc

@@ -109,9 +109,8 @@ void MimeSniffingURLLoader::OnComplete(
   DCHECK(!complete_status_.has_value());
   switch (state_) {
     case State::kWaitForBody:
-      // OnComplete() is called without OnStartLoadingResponseBody(). There is
-      // no response body in this case. Use |kDefaultMimeType| as its mime type
-      // even though it's empty.
+      // An error occured before receiving any data.
+      DCHECK_NE(net::OK, status.error_code);
       state_ = State::kCompleted;
       response_head_.mime_type = kDefaultMimeType;
       if (!throttle_) {
@@ -231,19 +230,9 @@ void MimeSniffingURLLoader::OnBodyWritable(MojoResult) {
 void MimeSniffingURLLoader::CompleteSniffing() {
   DCHECK_EQ(State::kSniffing, state_);
   if (buffered_body_.empty()) {
-    // A data pipe for the body was received but no body was provided. Don't
-    // propagate OnStartLoadingResponseBody() in this case. We treat this
-    // situation as the same as when OnStartLoadingResponseBody() was not
-    // called.
-    //
-    // TODO(crbug.com/826868): Remove this once all loaders are aligned.
-    state_ = State::kWaitForBody;
-    if (complete_status_.has_value()) {
-      auto status = complete_status_.value();
-      complete_status_.reset();
-      OnComplete(status);
-    }
-    return;
+    // The URLLoader ended before sending any data. There is not enough
+    // informations to determine the MIME type.
+    response_head_.mime_type = kDefaultMimeType;
   }
 
   state_ = State::kSending;
@@ -272,7 +261,13 @@ void MimeSniffingURLLoader::CompleteSniffing() {
   // Call OnComplete() if OnComplete() has already been called.
   if (complete_status_.has_value())
     destination_url_loader_client_->OnComplete(complete_status_.value());
+
+  if (bytes_remaining_in_buffer_) {
     SendReceivedBodyToClient();
+    return;
+  }
+
+  CompleteSending();
 }
 
 void MimeSniffingURLLoader::CompleteSending() {