Add Net.ProxyScriptFetchSuccessDuration UMA to record how long PAC script fetches take.

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 4dbeb5115a0bbbeeec867d6f2fbe4cc7bb77e539..5035e1518e32fd37525ab10c061a381ef400b812 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -21765,6 +21765,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>The length of the URL that was passed into the PAC script.</summary>
 </histogram>
 
+<histogram name="Net.ProxyScriptFetchSuccessDuration" units="milliseconds">

[Alexei Svitkine (slow), 2015/07/10 16:22:35]

Nit: Optional suggestion:

Name it: Net.ProxyScript.FetchSuccessDuration

Makes a bit more readable and lets you later add more metrics related to proxy
script while keeping a structure. (If you make the change, make sure to change
both the code and the xml).

+  <owner>cbentzel@chromium.org</owner>
+  <summary>
+    The time taken to successfully fetch a PAC script. This does not include
+    time spent doing proxy auto-discovery, or failed attempts at retrieiving PAC
+    scripts.
+  </summary>
+</histogram>
+
 <histogram name="Net.ProxyService.GetProxyUsingScriptTime"
     units="100s of microseconds">
   <owner>eroman@chromium.org</owner>