Add a use counter and histogram to FileReaderSync.

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 bbb8b8624d8e7f2bc3045f08a181c70da6a4ff40..a9748b113129e39687e72425330bd5b07449d04e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -19272,6 +19272,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="FileReaderSync.WorkerType" enum="FileReaderSyncWorkerType">
+  <owner>mek@chromium.org</owner>
+  <summary>
+    For each FileReaderSync instance records the type of the web worker that
+    created the instance.
+  </summary>
+</histogram>
+
 <histogram name="FileSystem.DirectoryDatabaseInit"
     enum="FileSystemDatabaseInitResult">
   <owner>tzik@chromium.org</owner>
@@ -89922,6 +89930,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1803" label="ServiceWorkerNavigationPreload"/>
   <int value="1805"
       label="HTMLIFrameElementAllowfullscreenAttributeSetAfterContentLoad"/>
+  <int value="1812" label="V8FileReaderSync_Constructor"/>
 </enum>
 
 <enum name="FetchRequestMode" type="int">
@@ -90761,6 +90770,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="6" label="File could not be exclusively opened."/>
 </enum>
 
+<enum name="FileReaderSyncWorkerType" type="int">
+  <int value="0" label="Other"/>
+  <int value="1" label="Dedicated Worker"/>
+  <int value="2" label="Shared Worker"/>
+  <int value="3" label="Service Worker"/>
+</enum>
+
 <enum name="FileSystemDatabaseInitResult" type="int">
   <int value="0" label="OK"/>
   <int value="1" label="Corruption"/>