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

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index a9161fccf87273ba6849d47cc82dfe9844199e0b..1a92d00f153308c384cbeea713810784652cf36b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -33959,6 +33959,31 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.JobControllerSet.CountOfAltJob" units="alt_jobs">
+  <owner>zhongyi@chromium.org</owner>
+  <summary>The counts number of alternative jobs which are uncleaned.</summary>

[xunjieli, 2017/03/17 00:47:56]

nit: "The counts number of" -> "The number of"  
and below

[Zhongyi Shi, 2017/03/17 02:02:30]

Done.

+</histogram>
+
+<histogram name="Net.JobControllerSet.CountOfJobController"
+    units="job_controllers">
+  <owner>zhongyi@chromium.org</owner>
+  <summary>The counts number of job controllers which are uncleaned.</summary>
+</histogram>
+
+<histogram name="Net.JobControllerSet.CountOfMainJob" units="main_jobs">
+  <owner>zhongyi@chromium.org</owner>
+  <summary>The counts number of main jobs which are uncleaned.</summary>
+</histogram>
+
+<histogram name="Net.JobControllerSet.CountOfPreconnect"
+    units="job_controllers">
+  <owner>zhongyi@chromium.org</owner>
+  <summary>
+    The counts number of job controllers which are used for preconnect and
+    uncleaned.

[xunjieli, 2017/03/17 00:47:56]

nit: "uncleaned" -> "are uncleaned"

[Zhongyi Shi, 2017/03/17 02:02:29]

Done.

+  </summary>
+</histogram>
+
 <histogram name="Net.LoadPrefetch.Pattern" enum="PrefetchStatus">
   <owner>droger@chromium.org</owner>
   <owner>mattcary@chromium.org</owner>