Document UMA histograms for app launcher discoverability.

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 977e434e879b4d648003c9d3400c327196a90463..564256e7c99cde3c0a8294fe50c56e6dda67aad2 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -196,6 +196,15 @@ other types of suffix sets.
   <summary>Interactions with the App Launcher promo dialog.</summary>
 </histogram>
 
+<histogram name="Apps.AppListHowEnabled" enum="AppListEnableSource">
+  <summary>
+    The trigger that caused the app list to be enabled. Recorded when the user
+    first shows the app list. If not shown after one hour, will be recorded
+    then. If Chrome was not running at the one-hour mark, will be recorded
+    during the next Chrome startup.
+  </summary>
+</histogram>
+
 <histogram name="Apps.AppListSearchCommenced" units="searches">
   <summary>
     The number of searches that are started in the app list. This is gathered
@@ -210,6 +219,16 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Apps.AppListTimeToDiscover" units="milliseconds">
+  <summary>
+    Time between enabling the app list, and a user explicitly choosing to show
+    it. If the app list is not shown after one hour, an entry in the last bucket
+    is recorded. If the user installs a second packaged app within one hour, or
+    if the app list was not enabled by installing a packaged app from the Web
+    Store, no time value is recorded - only Apps.AppListHowEnabled.
+  </summary>
+</histogram>
+
 <histogram name="Ash.ActiveTouchPoints">
   <summary>
     Number of active touch-points when a new touch-point is added.
@@ -22612,6 +22631,15 @@ other types of suffix sets.
   <int value="3" label="Learn more"/>
 </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"/>
+  <int value="2" label="Clicked app launcher link from the Web Store"/>
+  <int value="3" label="Command line flag"/>
+  <int value="4" label="Chrome reinstalled over old, enabled profile"/>
+  <int value="5" label="Second packaged app installed without showing"/>
+</enum>
+
 <enum name="AppListSearchResult" type="int">
   <int value="0" label="OMNIBOX"/>
   <int value="1" label="APP"/>