Add metrics to track buttons clicked in IME menu tray.

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 456e5f4a83e30f559265773946e25da076915bbf..7dc6ed60e141454a13b69ca0fe986b601a3387b7 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -21442,6 +21442,30 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="InputMethod.ImeMenu.EmojiButton">
+  <owner>azurewei@chromium.org</owner>
+  <owner>shuchen@chromium.org</owner>
+  <summary>
+    The number of times users click emoji button in opt-in IME menu.
+  </summary>
+</histogram>
+
+<histogram name="InputMethod.ImeMenu.HandwritingButton">
+  <owner>azurewei@chromium.org</owner>
+  <owner>shuchen@chromium.org</owner>
+  <summary>
+    The number of times users click handwriting button in opt-in IME menu.
+  </summary>
+</histogram>
+
+<histogram name="InputMethod.ImeMenu.VoiceButton">
+  <owner>azurewei@chromium.org</owner>
+  <owner>shuchen@chromium.org</owner>
+  <summary>
+    The number of times users click voice button in opt-in IME menu.
+  </summary>
+</histogram>
+
 <histogram name="InputMethod.ImeSwitch" enum="IMESwitchType">
   <owner>shuchen@chromium.org</owner>
   <summary>The trigger type of input method switches by user.</summary>