Document UMA histograms for app launcher discoverability.

To track effectiveness of the app launcher install education pages in
enabling users to find, and start using, the app launcher when it is
auto-enabled upon installing the first v2 packaged app from the Web
Store.

Histogram recording committed in http://crrev.com/250348.

BUG=314235

Review URL: https://codereview.chromium.org/156963005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251254 0039d316-1c4b-4281-b951-d872f2087c98

tools/metrics/histograms/histograms.xml

@@ -203,6 +203,15 @@ 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
@@ -217,6 +226,16 @@ 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.
@@ -22743,6 +22762,15 @@ 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"/>