Implement chrome://histograms with network service.

content/browser/histogram_internals_url_loader.cc

+// Copyright 2017 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 "content/browser/histogram_internals_url_loader.h"
+
+#include "base/metrics/histogram_macros.h"
+#include "base/metrics/statistics_recorder.h"
+#include "content/browser/histogram_internals_request_job.h"
+#include "mojo/common/data_pipe_utils.h"
+
+namespace content {
+
+void StartHistogramInternalsURLLoader(const ResourceRequest& request,
+                                      mojom::URLLoaderClientPtr client) {
+  scoped_refptr<net::HttpResponseHeaders> headers(
+      new net::HttpResponseHeaders("HTTP/1.1 200 OK"));
+  ResourceResponseHead resource_response;
+  resource_response.headers = headers;
+  resource_response.mime_type = "text/html";
+  client->OnReceiveResponse(resource_response, base::nullopt, nullptr);
+
+  base::StatisticsRecorder::ImportProvidedHistograms();
+  std::string data = HistogramInternalsRequestJob::GenerateHTML(request.url);
+
+  MojoCreateDataPipeOptions options;
+  options.struct_size = sizeof(MojoCreateDataPipeOptions);
+  options.flags = MOJO_CREATE_DATA_PIPE_OPTIONS_FLAG_NONE;
+  options.element_num_bytes = 1;
+  options.capacity_num_bytes = data.size();
+  mojo::DataPipe data_pipe(options);
+
+  DCHECK(data_pipe.producer_handle.is_valid());
+  DCHECK(data_pipe.consumer_handle.is_valid());
+
+  CHECK(mojo::common::BlockingCopyFromString(data, data_pipe.producer_handle));

[yzshen1, 2017/05/10 15:13:43]

The data pipe API doesn't guarantee that we could successfully write in one
shot. (Although the current implementation may always succeed for reasonable
size of data.) And we probably don't want any potentially blocking operation on
the UI thread.

It may be fine temporarily, but we (the mojo team) should develop easy-to-use,
work-in-all-case data pipe read/write helper classes so that users don't need to
worry about this.

[jam, 2017/05/10 15:29:58]

Re blocking, I think it's fine for testing pages like this. 
Agree, seems like this method should be it?

+
+  client->OnStartLoadingResponseBody(std::move(data_pipe.consumer_handle));
+
+  ResourceRequestCompletionStatus request_complete_data;
+  request_complete_data.error_code = net::OK;
+  request_complete_data.exists_in_cache = false;
+  request_complete_data.completion_time = base::TimeTicks::Now();
+  request_complete_data.encoded_data_length = data.size();
+  request_complete_data.encoded_body_length = data.size();
+  client->OnComplete(request_complete_data);
+}
+
+}  // namespace content