Add UMA for service worker navigation preload.

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 b8ed3e9575857bf9e225b2e331cd8ee86875f5cd..1cfd11a6c86cd9d55c08fbee8b4aee0818c55d7e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -59535,6 +59535,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ServiceWorker.ActivatedWorkerPreparationForMainFrame.Type"
+    enum="ServiceWorkerPreparationType">
+  <owner>falken@chromium.org</owner>
+  <summary>
+    The type of preparation needed for the browser to find and possibly start an
+    active worker to dispatch a FetchEvent for a main frame resource request.
+    See details at ServiceWorker.ActivatedWorkerPreparationForMainFrame.Time.
+    The suffixed histograms for .Time record the time required for each type.
+  </summary>
+</histogram>
+
 <histogram name="ServiceWorker.ActivateEvent.Time" units="ms">
   <owner>shimazu@chromium.org</owner>
   <summary>
@@ -104064,6 +104075,24 @@ value.
   <int value="20" label="NAVIGATION_HINT_LINK_TAP_DOWN"/>
 </enum>
 
+<enum name="ServiceWorkerPreparationType" type="int">
+  <summary>
+    The type of preparation that was required for the browser to find and
+    possibly start up a service worker to dispatch a fetch event to. From
+    ServiceWorkerMetrics::WorkerPreparationType.
+  </summary>
+  <int value="0" label="Unknown"/>
+  <int value="1"
+      label="The worker was already starting up. The browser waited for it
+             finish."/>
+  <int value="2" label="The worker was already running."/>
+  <int value="3"
+      label="The worker was stopping. Once stopped, the browser started it up"/>
+  <int value="4" label="The worker started up in a new process."/>
+  <int value="5" label="The worker started up in an existing process."/>
+  <int value="6" label="The worker started up during browser startup."/>
+</enum>
+
 <enum name="ServiceWorkerReadResponseResult" type="int">
   <int value="0" label="OK"/>
   <int value="1" label="Read headers error"/>
@@ -112744,6 +112773,16 @@ value.
   <affected-histogram name="Navigation.TimeToURLJobStart"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="NavigationPreloadEnabled">
+  <affected-histogram
+      name="ServiceWorker.ActivatedWorkerPreparationForMainFrame.Time"/>
+  <affected-histogram
+      name="ServiceWorker.ActivatedWorkerPreparationForMainFrame.Time_StartWorkerExistingProcess"/>
+  <suffix name="NavigationPreloadEnabled" label="Navigation preload occurred."/>
+  <affected-histogram
+      name="ServiceWorker.ActivatedWorkerPreparationForMainFrame.Type"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="Net.BidirectionalStreamExperiment" separator=".">
   <owner>xunjieli@chromium.org</owner>
   <suffix name="QUIC" label="Bidirectional streams that use QUIC protocol"/>