Write loading signal extra data in JSON

chrome/browser/android/offline_pages/prerendering_loader.cc

 // Copyright 2016 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/browser/android/offline_pages/prerendering_loader.h"
 
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/threading/thread_task_runner_handle.h"
@@ skipping 76 matching lines @@
 
 PrerenderingLoader::~PrerenderingLoader() {
   CancelPrerender();
 }
 
 void PrerenderingLoader::MarkLoadStartTime() {
   load_start_time_ = base::TimeTicks::Now();
 }
 
 void PrerenderingLoader::AddLoadingSignal(const char* signal_name) {
-  std::string signal(signal_name);
-  signal += ": ";
   base::TimeTicks current_time = base::TimeTicks::Now();
   base::TimeDelta delay_so_far = current_time - load_start_time_;
-  signal += std::to_string(delay_so_far.InMilliseconds());
-  signal_data_.push_back(signal);
+  double delay = delay_so_far.InMilliseconds();

[romax, 2017/04/18 00:35:09]

TimeDelta::InMillisecondsF returns a double, while the current one is returning
an int64_t. More like a nit but why we're using double over int64_t?

[Pete Williamson, 2017/04/18 18:26:08]

It turns out that double is an acceptable JSON type, but int64_t is not
(JavaScript doesn't know what to do with it.)  The doc in values.h suggests that
we use double if we need an int64_t.  In this case, especially since we are
subtracting off the times to get a delta in milliseconds, we are not in imminent
danger of exceeding the precision offered by a double, and losing any precision.

+  signal_data_.SetDouble(signal_name, delay);
 }
 
 bool PrerenderingLoader::LoadPage(const GURL& url,
                                   const LoadPageCallback& load_done_callback,
                                   const ProgressCallback& progress_callback) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
   if (!IsIdle()) {
     DVLOG(1)
         << "WARNING: Existing request in progress or waiting for StopLoading()";
     return false;
@@ skipping 182 matching lines @@
   if (adapter_->IsActive()) {
     adapter_->DestroyActive();
   }
   snapshot_controller_.reset(nullptr);
   session_contents_.reset(nullptr);
   state_ = State::IDLE;
   is_lowbar_met_ = false;
 }
 
 }  // namespace offline_pages