Add metrics for calculating precision/recall of link navigation pre-connect triggers.

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 e5df0e687581e38dbd2f6ef21468d1a5e3dc8cb5..abcc389952e73673f58fd747dc6c36add1f82606 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -5338,9 +5338,9 @@ other types of suffix sets.
 
 <histogram name="Net.PreconnectedNavigation" enum="PreconnectedNavigation">
   <summary>
-    Indicate whether the navigation was preceded by a recent pre-connect
-    (pre-connect within 10 seconds). There is a high chance that loading the
-    page used a preconnected TCP session.
+    Indicate whether a link navigation was preceded by a recent pre-connect
+    trigger (within 10 seconds). There is a high chance that loading the page
+    used a preconnected TCP session.
   </summary>
 </histogram>
 
@@ -5393,6 +5393,14 @@ other types of suffix sets.
   </details>
 </histogram>
 
+<histogram name="Net.PreconnectTriggerused" enum="PreconnectTriggerUsed">
+  <summary>
+    Indicate whether if a preconnect trigger is followed by a resource request
+    (from link navigations) to the host or not. This is to measure precision of
+    link-based preconnect triggers.
+  </summary>
+</histogram>
+
 <histogram name="Net.PreconnectUtilization" enum="NetPreconnectUtilization">
   <obsolete>
     Sourced data corrected, and replaced by NetPreconnectUtilization2
@@ -17067,8 +17075,8 @@ other types of suffix sets.
 </enum>
 
 <enum name="PreconnectedNavigation" type="int">
-  <int value="0" label="Page nav. preceded by a pre-connect"/>
-  <int value="1" label="No recent pre-connect to the page"/>
+  <int value="0" label="No recent pre-connect to the page"/>
+  <int value="1" label="Page nav. preceded by a pre-connect"/>
 </enum>
 
 <enum name="PreconnectMotivation" type="int">
@@ -17091,6 +17099,11 @@ other types of suffix sets.
   <int value="2" label="TOO_NEW"/>
 </enum>
 
+<enum name="PreconnectTriggerUsed" type="int">
+  <int value="0" label="The pre-connect triggered host was not accessed"/>
+  <int value="1" label="The pre-connect triggered host was accessed"/>
+</enum>
+
 <enum name="PrerenderFinalStatus" type="int">
   <int value="0" label="USED"/>
   <int value="1" label="TIMED_OUT"/>