Improve the resolution of the menu item homescreen launch source metric.

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 f6a3aa9a7c846f867e1938c5f5fa3dfc50f86021..e48c3927b8a3bc29227dce3d764785b62d1c92b8 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -23423,6 +23423,19 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>
     Records the source of an Android homescreen launch intent used to launch
     Chrome.
+
+    The Menu item bucket is deprecated as of M57, and will only contain records
+    up until M56.
+
+    For M57+, the Menu item (standalone) bucket contains all launch intents
+    originating from the add to homescreen menu item that launch to standalone
+    mode and are not PWAs. It also contains all such intents that were added to
+    the homescreen *prior* to M57, as we cannot determine which of these are or
+    are not PWAs.
+
+    For M57+, the Menu item (shortcut) bucket contains all launch intents
+    originating from the add to homescreen menu item that launch to a browser
+    tab.
   </summary>
 </histogram>
 
@@ -96115,11 +96128,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 
 <enum name="LaunchFromHomeScreenSource" type="int">
   <int value="0" label="Unknown"/>
-  <int value="1" label="Menu item"/>
+  <int value="1" label="Menu item (deprecated in M57+)"/>
   <int value="2" label="App banner"/>
   <int value="3" label="Bookmark navigator widget"/>
   <int value="4" label="Bookmark shortcut widget"/>
   <int value="5" label="Notification"/>
+  <int value="6" label="Menu item (PWA)"/>
+  <int value="7" label="Menu item (standalone)"/>
+  <int value="8" label="Menu item (shortcut)"/>
 </enum>
 
 <enum name="LaunchIntentFlags" type="int">