Add histograms for prerender rel types.

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 7e64e5b0cf6a4cd441e228a1df92875a80100d06..a22e9736be356986534ec9146be4ce93022d8a2b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -15348,6 +15348,20 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Prerender.RelTypesLinkAdded" enum="PrerenderRelTypes">
+  <summary>
+    For each prerender link added to a document, records the rel types present
+    on the link element.
+  </summary>
+</histogram>
+
+<histogram name="Prerender.RelTypesLinkStarted" enum="PrerenderRelTypes">
+  <summary>
+    For each prerender in a document which starts prerendering, records the rel
+    types present on the link element.
+  </summary>
+</histogram>
+
 <histogram name="Prerender.RendererIdleTime" units="milliseconds">
   <obsolete>
     deprecated Nov 16 2012
@@ -29996,6 +30010,13 @@ other types of suffix sets.
   <int value="3" label="PAGEVIEW_EVENT_TOP_SITE_LOAD_START"/>
 </enum>
 
+<enum name="PrerenderRelTypes" type="int">
+  <int value="0" label="PRERENDER_REL_TYPE_NONE"/>
+  <int value="1" label="PRERENDER_REL_TYPE_PRERENDER"/>
+  <int value="2" label="PRERENDER_REL_TYPE_NEXT"/>
+  <int value="3" label="PRERENDER_REL_TYPE_PRERENDER_AND_NEXT"/>

[Ilya Sherman, 2014/02/07 21:32:59]

Optional nit: These labels show as human-readable text, so you might want to
format these in a more human-friendly way.

+</enum>
+
 <enum name="PrerenderSchemeCancelReason" type="int">
   <int value="0" label="EXTERNAL_PROTOCOL"/>
   <int value="1" label="DATA"/>