Add experiment to exercise AffiliationService with dummy data, plus add related UMA histograms.

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 c85f1160cb4034e8aa0ca73123049098b842dbd4..d3a55a4081cf9b6956631191980d8f7531ed9346 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -247,6 +247,50 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="AffiliationService.DummyData.RequestResultCount"
+    units="results">
+  <owner>engedy@chromium.org</owner>
+  <summary>
+    The number of facets affiliated with each of the dummy Web facet, as per
+    information available in the cache after preloading affiliations for the
+    corresponding dummy Android facets.
+  </summary>
+</histogram>
+
+<histogram name="AffiliationService.DummyData.RequestSuccess"
+    enum="BooleanSuccess">
+  <owner>engedy@chromium.org</owner>
+  <summary>
+    Whether or not getting affiliations for each of the dummy Web facets from
+    the cache has succeeded after preloading affiliations for the corresponding
+    dummy Android facets into the cache.

[Ilya Sherman, 2015/04/21 00:38:05]

Boy, I find this sentence hard to parse.  Is this recorded once per dummy Web
facet, or once for the aggregate?  Maybe you could split it into multiple
sentences for improved clarity?

[engedy, 2015/04/21 18:29:07]

Done.

+  </summary>
+</histogram>
+
+<histogram name="AffiliationService.FetchSize" units="facets">
+  <owner>engedy@chromium.org</owner>
+  <summary>
+    The number of facets fetched in a single request by the AffiliationService,
+    recorded for every request.

[Ilya Sherman, 2015/04/21 00:38:05]

nit: The histogram descriptions might be easier to read if you split off the
"recorded upon ..." information into a separate sentence.  (Applies to all of
the histograms)

[engedy, 2015/04/21 18:29:08]

Done.

+  </summary>
+</histogram>
+
+<histogram name="AffiliationService.FirstFetchDelay" units="milliseconds">
+  <owner>engedy@chromium.org</owner>
+  <summary>
+    The time elapsed between start-up and the first time the AffiliationService
+    needs to issue a network fetch.

[Ilya Sherman, 2015/04/21 00:38:05]

nit: "needs" -> "needed"

[engedy, 2015/04/21 18:29:08]

Done.

+  </summary>
+</histogram>
+
+<histogram name="AffiliationService.SubsequentFetchDelay" units="milliseconds">
+  <owner>engedy@chromium.org</owner>
+  <summary>
+    The elapsed time between subsequent network fetches issued by the
+    AffiliationService.
+  </summary>
+</histogram>
+
 <histogram name="Android.KernelVersion" enum="AndroidKernelVersion">
   <owner>rsesek@chromium.org</owner>
   <summary>