Add "Search" key press/hold UMA metrics.

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 47f4d39d47ace9034bb182298b35e380f1b64186..08ee024e9cb6d9922a79fecb9c33cfe9df821028 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -23453,6 +23453,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Keyboard.Shortcuts.CrosSearchKeyDelay" units="ms">
+  <owner>xiaohuic@chromium.org</owner>
+  <owner>zalcorn@chromium.org</owner>
+  <summary>
+    The time between a user pressing the ChromeOS &quot;Search&quot; key and the
+    second key being pressed down to active a shortcut. e.g. Search + L would
+    record the time between Search key down and L key down. If the user pressed
+    multiple keys after Search, we only record the first key down.
+  </summary>
+</histogram>
+
 <histogram name="Kiosk.LaunchType" enum="KioskLaunchType">
   <owner>xiyuan@chromium.org</owner>
   <owner>blumberg@chromium.org</owner>