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 224814a18431e6aa6824392aac58e2e62d33157c..79a0c0cd267d418d678424df8ddd29f5a439436c 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -21634,6 +21634,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="InputMethod.ImeMenu.EmojiHandwritingVoiceButton"
+    enum="ImeMenuButtonType">
+  <owner>azurewei@chromium.org</owner>
+  <owner>shuchen@chromium.org</owner>
+  <summary>
+    The number of times users click emoji, handwriting or 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>
@@ -91578,6 +91588,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </int>
 </enum>
 
+<enum name="ImeMenuButtonType" type="int">
+  <int value="0" label="Unknown"/>
+  <int value="1" label="Emoji"/>
+  <int value="2" label="Handwriting"/>
+  <int value="3" label="Voice"/>
+  <int value="4" label="Settings"/>
+</enum>
+
 <enum name="IMERegisterProxyView" type="int">
   <int value="0" label="Success">Success in registering ProxyView to IMM</int>
   <int value="1" label="Failure">Failure in registering ProxyView to IMM</int>