Add time-to-network histogram considering browser side navigation

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 a7aaf18c8969f7ea0dc741c4b83a046932e51f08..07b066b79af292a70d243192cea68011fea03e79 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -23921,6 +23921,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <summary>The result from an attempt to load a PPAPI plugin.</summary>
 </histogram>
 
+<histogram name="PlzNavigate.TimeToNetworkRequest" units="milliseconds">

[clamy, 2014/09/19 14:56:03]

We have a section of histograms called Navigation, so I suggest renaming it
Navigation.TimeToURLJobStart.

[carlosk, 2014/09/23 17:02:57]

Done.

+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Time delay from a navigation request until the first network request is

[clamy, 2014/09/19 14:56:03]

I think the following description is more accurate:
Time between the start of a navigation request in the browser and the reception
of a corresponding ResourceRequest in the network stack.

[carlosk, 2014/09/23 17:02:57]

Done.

+    about to be made.
+  </summary>
+</histogram>
+
 <histogram name="Power.BacklightLevelOnAC" units="%">
   <owner>derat@chromium.org</owner>
   <summary>