[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 45b254561762a6e180a25bff872cab9ab2e37a92..fa498080f9561ff2218414b3dfb0215a7ee6b0ef 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.
@@ -2208,6 +2212,15 @@ const Experiment kExperiments[] = {
     ENABLE_DISABLE_VALUE_TYPE(switches::kEnableChildAccountDetection,
                               switches::kDisableChildAccountDetection)
   },
+#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