[Default Apps] Add Stadia label to DefaultAppName.

Add the Stadia label to the DefaultAppName enum in enums.xml.

Bug: 1125414
Change-Id: I31a66972a0a7705e6b3bcac32b4674611fd9365b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2474079
Auto-Submit: Jeevan Shikaram <jshikaram@chromium.org>
Reviewed-by: dstockwell <dstockwell@google.com>
Commit-Queue: Jeevan Shikaram <jshikaram@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818340}

tools/metrics/histograms/enums.xml

@@ -15999,6 +15999,7 @@ metrics consent we also won't be able to send UMA metrics. -->
   <int value="36" label="Web Store"/>
   <int value="37" label="YouTube"/>
   <int value="38" label="YouTube Music"/>
+  <int value="40" label="Stadia"/>
 </enum>
 
 <enum name="DefaultBrowserAsyncAttemptResult">