Add histograms to track the number of job controller and job that are

net/http/http_stream_factory_impl.cc

Index: net/http/http_stream_factory_impl.cc
diff --git a/net/http/http_stream_factory_impl.cc b/net/http/http_stream_factory_impl.cc
index 0a742d1284204e4fcedf919df133dab72df2417f..271d4be508dd4f73f821e2ccd82c2eb014f6473d 100644
--- a/net/http/http_stream_factory_impl.cc
+++ b/net/http/http_stream_factory_impl.cc
@@ -104,6 +104,28 @@ HttpStreamFactoryImpl::HttpStreamFactoryImpl(HttpNetworkSession* session,
 HttpStreamFactoryImpl::~HttpStreamFactoryImpl() {
   DCHECK(request_map_.empty());
   DCHECK(spdy_session_request_map_.empty());
+  size_t alt_job_count = 0;
+  size_t main_job_count = 0;
+  size_t preconnect_controller_count = 0;

[Alexei Svitkine (slow), 2017/03/20 15:51:58]

Nit: Just use int since these are not compared with size_t types (like
vector.size()).

[xunjieli, 2017/03/20 16:37:35]

Is this written somewhere?
According to
https://chromium.googlesource.com/chromium/src/+/master/styleguide/c++/c++.md...
, for object counts, "size_t" is recommended.

[Alexei Svitkine (slow), 2017/03/20 16:56:04]

The normal C++ style guide suggests to use int types unless there's a reason not
to:
https://google.github.io/styleguide/cppguide.html#Integer_Types

In Chromium, often there's a reason to use size_t because those end up being
compared with standard library sizes - as I mentioned, but here it's not the
case. You only end up passing the values to the histogram macros, which end up
being a signed type (HistogramBase::Sample which is int32_t).

[Zhongyi Shi, 2017/03/20 21:26:35]

Done.

+  for (const auto& it : job_controller_set_) {
+    DCHECK(it->HasPendingAltJob() || it->HasPendingMainJob());
+    // For a preconnect controller, it should have exactly the main job.
+    if (it->is_preconnect()) {
+      preconnect_controller_count++;
+      continue;
+    }
+    // For non-preconnects.
+    if (it->HasPendingAltJob())
+      alt_job_count++;
+    if (it->HasPendingMainJob())
+      main_job_count++;
+  }
+  UMA_HISTOGRAM_COUNTS_1M("Net.JobControllerSet.CountOfPreconnect",
+                          preconnect_controller_count);
+  UMA_HISTOGRAM_COUNTS_1M("Net.JobControllerSet.CountOfNonPreconnectAltJob",
+                          alt_job_count);
+  UMA_HISTOGRAM_COUNTS_1M("Net.JobControllerSet.CountOfNonPreconnectMainJob",
+                          main_job_count);
 }
 
 HttpStreamRequest* HttpStreamFactoryImpl::RequestStream(