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

tools/metrics/histograms/histograms.xml

 <!--
 Copyright 2013 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <!--
 This file is used to generate a comprehensive list of Chrome histograms along
 with a detailed description for each histogram.
 
@@ skipping 5320 matching lines @@
 <histogram name="Net.OSErrorsForGetAddrinfo_Win"
     enum="ErrorCodesGetaddrinfo_Win">
   <summary>
     Positive error code that was returned by the system library
     &quot;getaddrinfo()&quot;.
   </summary>
 </histogram>
 
 <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>
 
 <histogram name="Net.PreconnectMotivation" enum="PreconnectMotivation">
   <summary>
     When a preconnection is made, indicate what the motivation was.
   </summary>
   <details>
     Currently, the most common (only?) motivations are SELF_REFERAL,
     LEARNED_REFERAL and OMNIBOX. The SELF_REFERAL indicates that we made sure a
@@ skipping 32 matching lines @@
     The expected number of connections, times 100, that we'll make to a given
     subresource, based on learned history.
   </summary>
   <details>
     By comparing this to thresholds, we decide if we will preconnect,
     preresolve, or do nothing. This histogram can be used to select those static
     thresholds.
   </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
   </obsolete>
   <summary>
     Indicate final utilization for each attempted socket connection.
   </summary>
   <details>
     We also include stats for non-speculative sockets. Some socket connections
     may never connect, and others may never be used (as the user may abort
@@ skipping 11654 matching lines @@
   <int value="5" label="5"/>
 </enum>
 
 <enum name="PowerBrightnessAdjust" type="int">
   <int value="0" label="Brightness Down"/>
   <int value="1" label="Brightness Up"/>
   <int value="2" label="Brightness Absolute"/>
 </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">
   <int value="0" label="MOUSE_OVER_MOTIVATED"/>
   <int value="1" label="PAGE_SCAN_MOTIVATED"/>
   <int value="2" label="UNIT_TEST_MOTIVATED"/>
   <int value="3" label="LINKED_MAX_MOTIVATED"/>
   <int value="4" label="OMNIBOX_MOTIVATED"/>
   <int value="5" label="STARTUP_LIST_MOTIVATED"/>
   <int value="6" label="EARLY_LOAD_MOTIVATED"/>
   <int value="7" label="NO_PREFETCH_MOTIVATION"/>
   <int value="8" label="STATIC_REFERAL_MOTIVATED"/>
   <int value="9" label="LEARNED_REFERAL_MOTIVATED"/>
   <int value="10" label="SELF_REFERAL_MOTIVATED"/>
 </enum>
 
 <enum name="PreconnectSubresourceEval" type="int">
   <int value="0" label="PRECONNECTION"/>
   <int value="1" label="PRERESOLUTION"/>
   <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"/>
   <int value="2" label="EVICTED"/>
   <int value="3" label="MANAGER_SHUTDOWN"/>
   <int value="4" label="CLOSED"/>
   <int value="5" label="CREATE_NEW_WINDOW"/>
   <int value="6" label="PROFILE_DESTROYED"/>
   <int value="7" label="APP_TERMINATING"/>
   <int value="8" label="JAVASCRIPT_ALERT"/>
@@ skipping 2822 matching lines @@
   <group name="Disabled" label="Neither extra webstore link is visible"/>
   <group name="FooterLink" label="Link in bottom right of footer"/>
   <group name="PlusIcon" label="Plus icon in apps page"/>
   <affected-histogram name="Extensions.AppLaunch"/>
   <affected-histogram name="NewTabPage.DefaultPageType"/>
 </fieldtrial>
 
 </fieldtrials>
 
 </histogram-configuration>