Add some basic UMA for proxy service.

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 1f7ad6f405204985db0851d3ca630e044b32d33f..8fd544236c9d3817f8b41de393598acacdd40c15 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -20344,6 +20344,41 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <summary>The length of the URL that was passed into the PAC script.</summary>
 </histogram>
 
+<histogram name="Net.ProxyService.GetProxyUsingScriptTime">
+  <owner>eroman@chromium.org</owner>
+  <summary>
+    The time taken to resolve a URL by forwarding the request to the proxy
+    resolver and executing the PAC script. This includes PAC script execution
+    and DNS queries, but does not include retries due to errors and changing

[eroman, 2015/05/11 23:42:41]

it "may" include time due to changing configuration. For instance:

   (1) start 4 PAC requests
   (2) ProxyConfigService notifies ProxyService of a configuration change
   (3) The PAC requests are cancelled
   (4) The new configuration is applied (maybe involves downloading a PAC script
and re-creating a proxy resolver)
   (5) The Request is re-submitted to the new ProxyResolver
   (6) It completes, and logs its time relative to Request::creation_time_

I think the metric you are logging is correct and useful -- that it includes the
extra time for re-fetching is good so it is directly comparable to
ResolveProxyTime.

My only concern is with the phrasing of this description. I suggest making it
more like that of ResoleProxyTime

[Anand Mistry (off Chromium), 2015/05/12 00:33:25]

Done.

+    configurations.
+  </summary>
+</histogram>
+
+<histogram name="Net.ProxyService.ResolvedUsingScript">
+  <owner>eroman@chromium.org</owner>
+  <summary>
+    Whether proxy resolution occured by forwarding the request to the proxy
+    resolver and executing the PAC script.
+  </summary>
+</histogram>
+
+<histogram name="Net.ProxyService.ResolveProxyTime">
+  <owner>eroman@chromium.org</owner>
+  <summary>
+    The total time taken to resolve a URL. THis includes PAC script execution,

[eroman, 2015/05/11 23:42:41]

typo: THis --> This

[Anand Mistry (off Chromium), 2015/05/12 00:33:25]

Done.

+    DNS queries, and retries.
+  </summary>
+</histogram>
+
+<histogram name="Net.ProxyService.ScriptTerminated">
+  <owner>eroman@chromium.org</owner>
+  <summary>
+    Whether a proxy resolution resulted in the PAC script terminating
+    unexpectedly. This happens when the resolver process crashes when using
+    out-of-process PAC.
+  </summary>
+</histogram>
+
 <histogram name="Net.PublicKeyPinFailureDomain" enum="PublicKeyPinFailedDomain">
   <owner>agl@chromium.org</owner>
   <summary>