Add metrics to track buttons clicked in IME menu tray.

ash/common/system/chromeos/ime_menu/ime_menu_tray.cc

Index: ash/common/system/chromeos/ime_menu/ime_menu_tray.cc
diff --git a/ash/common/system/chromeos/ime_menu/ime_menu_tray.cc b/ash/common/system/chromeos/ime_menu/ime_menu_tray.cc
index 2fd3d3f74625a7ff65c66fd7f1dc518d52bc18c6..a6f2988e405bda531f8c7e4f4213dc2de5ee2767 100644
--- a/ash/common/system/chromeos/ime_menu/ime_menu_tray.cc
+++ b/ash/common/system/chromeos/ime_menu/ime_menu_tray.cc
@@ -26,6 +26,7 @@
 #include "ash/common/wm_shell.h"
 #include "ash/common/wm_window.h"
 #include "ash/public/cpp/shell_window_ids.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/strings/utf_string_conversions.h"
 #include "grit/ash_resources.h"
 #include "grit/ash_strings.h"
@@ -202,14 +203,18 @@ class ImeButtonsView : public views::View,
     // extensions. InputMethodManager::ShowKeyboardWithKeyset() will deal with
     // the |keyset| string to generate the right input view url.
     std::string keyset;
-    if (sender == emoji_button_)
+    if (sender == emoji_button_) {
       keyset = "emoji";
-    else if (sender == voice_button_)
+      UMA_HISTOGRAM_COUNTS("InputMethod.ImeMenu.EmojiButton", 1);
+    } else if (sender == voice_button_) {
       keyset = "voice";
-    else if (sender == handwriting_button_)
+      UMA_HISTOGRAM_COUNTS("InputMethod.ImeMenu.VoiceButton", 1);
+    } else if (sender == handwriting_button_) {
       keyset = "hwt";
-    else
+      UMA_HISTOGRAM_COUNTS("InputMethod.ImeMenu.HandwritingButton", 1);

[Alexei Svitkine (slow), 2017/01/03 15:47:22]

UMA_HISTOGRAM_COUNTS allocates 50 buckets but you're only logging to bucket 1.
This is wasteful.

It would be more efficient to log an UMA_HISTOGRAM_BOOLEAN.

However, given you have 3 things that are related, you would be better served
making this an enum histogram, with the different cases as different enum
entries.

If you do that, suggest making a helper function to log the enum histogram so
that you don't repeat the macro - as each macro expands to a bunch of code and
thus multiple macro statements bloat the binary.

[Azure Wei, 2017/01/04 14:02:41]

Thanks. I've updated these with enum histogram and added a helper method
RecordButtonsClicked(). Please take another look. Thanks!

+    } else {
       NOTREACHED();
+    }
 
     ime_menu_tray_->ShowKeyboardWithKeyset(keyset);
   }