[Extensions Toolbar Mac] Enable the extension action redesign flag on mac

The extension action redesign has progressed far enough on mac to warrant
the switch being available.

BUG=429810

Review URL: https://codereview.chromium.org/836743003

Cr-Commit-Position: refs/heads/master@{#310025}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 60e30791..713f97b 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1966,7 +1966,7 @@
     "enable-extension-action-redesign",
     IDS_FLAGS_ENABLE_EXTENSION_ACTION_REDESIGN_NAME,
     IDS_FLAGS_ENABLE_EXTENSION_ACTION_REDESIGN_DESCRIPTION,
-    kOsWin | kOsLinux | kOsCrOS,
+    kOsDesktop,
     SINGLE_VALUE_TYPE(extensions::switches::kEnableExtensionActionRedesign)
   },
 #endif