Add UMA for number of outstanding requests in ResourceDispatcherHostImpl

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 1ab43ba4ac96b04d3adebe858532cb222c2ad8f9..03b48cea03af6d1de4906de15c660cc865ee1760 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -37202,6 +37202,24 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.ResourceDispatcherHost.OutstandingRequests.PerProcess"
+    units="requests">
+  <owner>xunjieli@chromium.org</owner>
+  <summary>
+    The largest number of outstanding requests that are handled by the resource
+    dispatcher host for a single process.
+  </summary>
+</histogram>
+
+<histogram name="Net.ResourceDispatcherHost.OutstandingRequests.Total"
+    units="requests">
+  <owner>xunjieli@chromium.org</owner>
+  <summary>
+    The largest number of outstanding requests that are handled by the resource
+    dispatcher host across all processes.
+  </summary>
+</histogram>
+
 <histogram name="Net.ResourceLoader.ExpectedContentSizeResult"
     enum="ResourceLoaderExpectedContentSizeResult">
   <owner>maksim.sisov@intel.com</owner>