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..07133b8e098719d9dfa0b02efda8645653566a3a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -20344,6 +20344,45 @@ 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">

[Mark P, 2015/05/12 04:21:42]

units=?
(here and elsewhere, though elsewhere some may be enum=)

Perhaps milliseconds for this histogram.

[Anand Mistry (off Chromium), 2015/05/12 05:13:24]

I think it's 100s of microseconds since it uses a custom time scale (us is too
fine, ms is too coarse).  Not exactly sure how to express this.

I should mention that the timing histograms exist because we want to compare
before/after, and not necessarily look at absolute values.

+  <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,
+    DNS queries, and internal retries. This does not include retries initiated
+    by the user, such as by calling ProxyService::ReconsiderProxyAfterError().
+  </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. Note that in some cases, the system

[Mark P, 2015/05/12 04:21:42]

Please say explicitly when this is recorded.  (Does every URL request get passed
through proxy resolution?)

[Anand Mistry (off Chromium), 2015/05/12 05:13:24]

Done.

+    proxy resolver is used instead and this counts the requests forwarded to the

[Mark P, 2015/05/12 04:21:42]

this?
If so,
this counts
->
this histogram includes
and perhaps
is used
->
is used internally

[Anand Mistry (off Chromium), 2015/05/12 05:13:24]

Done.

+    system proxy resolver. However, on desktop platforms, this should be a small

[Mark P, 2015/05/12 04:21:42]

this = ? the count of all emissions to the histogram?  the count of all times
the system proxy resolver is used?  the count of all emissions to the histogram
for the bucket ____?

[Anand Mistry (off Chromium), 2015/05/12 05:13:24]

Done.

+    number of users since the default is to use V8.
+  </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,
+    DNS queries, and internal retries. This does not include retries initiated
+    by the user, such as by calling ProxyService::ReconsiderProxyAfterError().
+  </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.

[Mark P, 2015/05/12 04:21:42]

is this the only time it can occur?

[Anand Mistry (off Chromium), 2015/05/12 05:13:24]

Yes.

+  </summary>
+</histogram>
+
 <histogram name="Net.PublicKeyPinFailureDomain" enum="PublicKeyPinFailedDomain">
   <owner>agl@chromium.org</owner>
   <summary>