Add enum value for "sync" WebAppInstallSource in metrics enums.xml.

It now matches WebAppInstallSource enum in installable_metrics.h and
will show the label in the UKM dashboard.
The "sync" value wasn't recorded before but now is used in
WebApp.DailyInteraction UKM.

Change-Id: I050ae705e17070de21eeabbe130c7d560d29b1b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2245989Reviewed-by: Glen Robertson <glenrob@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Auto-Submit: Glen Robertson <glenrob@chromium.org>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779200}

tools/metrics/histograms/enums.xml

@@ -71259,6 +71259,7 @@ Full version information for the fingerprint enum values:
   <int value="13" label="Policy-install on Chrome OS"/>
   <int value="14" label="System default app on Chrome OS"/>
   <int value="15" label="Install icon in the Omnibox"/>
+  <int value="16" label="Installed from sync"/>
 </enum>
 
 <enum name="WebAppLauncherLaunchResult">