Enable feature DesktopPWAsElidedExtensionsMenu by default on Chrome OS.

This CL:
 - Hides the Extensions icon in web app windows unless the Extensions
   menu is opened.
 - Adds an "Extensions" menu item to the web app three dot menu to
   open the Extensions menu.

Bug: 1155421
Change-Id: I474a6ced0e7dd7893317bff4d4b1d8256957d80b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2620721
Commit-Queue: Matt Giuca <mgiuca@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842249}

chrome/common/chrome_features.cc

@@ -241,7 +241,13 @@ const base::Feature kDesktopPWAsCacheDuringDefaultInstall{
 // Moves the Extensions "puzzle piece" icon from the title bar into the app menu
 // for web app windows.
 const base::Feature kDesktopPWAsElidedExtensionsMenu{
-    "DesktopPWAsElidedExtensionsMenu", base::FEATURE_DISABLED_BY_DEFAULT};
+  "DesktopPWAsElidedExtensionsMenu",
+#if BUILDFLAG(IS_CHROMEOS_ASH) || BUILDFLAG(IS_CHROMEOS_LACROS)
+      base::FEATURE_ENABLED_BY_DEFAULT
+#else
+      base::FEATURE_DISABLED_BY_DEFAULT
+#endif
+};
 
 // Replaces the origin text flash in web app titlebars with the name of the app.
 const base::Feature kDesktopPWAsFlashAppNameInsteadOfOrigin{