Add UMA for the app list doodle being clicked.

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 aac1a84b9ee382def836070222f4aaf31fb5401c..52f28bd4260f767049f08ecc34eabeb212f34261 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -465,6 +465,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Apps.AppListDoodleAction" enum="AppListDoodleAction">
+  <owner>calamity@chromium.org</owner>
+  <summary>
+    The number of user interactions with the app list doodle. This is logged
+    once per action.
+  </summary>
+</histogram>
+
 <histogram name="Apps.AppListHowEnabled" enum="AppListEnableSource">
   <owner>tapted@chromium.org</owner>
   <summary>
@@ -43692,6 +43700,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="18" label="SOURCE_TEST"/>
 </enum>
 
+<enum name="AppListDoodleAction" type="int">
+  <int value="0" label="DOODLE_SHOWN"/>
+  <int value="1" label="DOODLE_CLICKED"/>
+</enum>
+
 <enum name="AppListEnableSource" type="int">
   <int value="0" label="Not enabled (should never be recorded)"/>
   <int value="1" label="Packaged app installed from Web Store"/>