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..16da7312bc0b73b3a9069e7bb75ea9c360f2f17a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -20344,6 +20344,40 @@ 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,
+    DNS queries, and retries.

[eroman, 2015/05/12 00:48:51]

nit: retries is somewhat ambiguous. There is another mechanism for retries
(ReconsiderProxyAfterError) which is not counted by this statistic (each attempt
will be logged separately). Again I think what is being measured is fine, but
the wording here might benefit from further tweaks.

[Anand Mistry (off Chromium), 2015/05/12 03:07:25]

I've changed the wording a bit to clarify that these are retries that happen
automatically. Calling ReconsiderProxyAfterError is a user-initiated retry. From
what I can tell, this metric captures all retries that happen without any user
intervention.

+  </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.

[eroman, 2015/05/12 00:48:50]

nit: there are exceptions: if the --winhttp-proxy-resolver is used, then this
will actually be measuring the time to evaluate proxy using WinHTTP when on
windows, and surprisingly also changes the meaning on Mac (even though it
doesn't use WinHTTP). Moreover when running in --single-process mode the meaning
is equivalent.

Perhaps not necessary to get into the full details, but I felt the need to point
this out.

[Anand Mistry (off Chromium), 2015/05/12 03:07:25]

I've added some wording to point this out, without getting into too many
details.

+  </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 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>