[ozone] Add an about flag to enable ozone overlay testing.

chrome/browser/about_flags.cc

Index: chrome/browser/about_flags.cc
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 7f41b7f2bcae79c6519c8e376d453503e43fae94..de1e90b8dec96c685cdeeaa440461c923d0b1347 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -66,6 +66,10 @@
 #include "extensions/common/switches.h"
 #endif
 
+#if defined(USE_OZONE)
+#include "ui/ozone/public/ozone_switches.h"
+#endif
+
 namespace about_flags {
 
 // Macros to simplify specifying the type.
@@ -2185,6 +2189,15 @@ const Experiment kExperiments[] = {
         switches::kSyncServiceURL,
         "https://chrome-sync.sandbox.google.com/chrome-sync/alpha")
   },
+#if defined(OS_CHROMEOS) && defined(USE_OZONE)
+  {
+    "ozone-test-single-overlay-support",
+    IDS_FLAGS_OZONE_TEST_SINGLE_HARDWARE_OVERLAY,
+    IDS_FLAGS_OZONE_TEST_SINGLE_HARDWARE_OVERLAY_DESCRIPTION,
+    kOsCrOS,
+    SINGLE_VALUE_TYPE(switches::kOzoneTestSingleOverlaySupport)
+  },
+#endif  // defined(OS_CHROMEOS) && defined(USE_OZONE)
 
   // NOTE: Adding new command-line switches requires adding corresponding
   // entries to enum "LoginCustomFlags" in histograms.xml. See note in