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

BUG=

Review URL: https://codereview.chromium.org/967733005

Cr-Commit-Position: refs/heads/master@{#318964}

chrome/app/generated_resources.grd

@@ -6886,6 +6886,14 @@ Keep your key file in a safe place. You will need it to create new versions of y
       <message name="IDS_FLAGS_SYNC_SANDBOX_DESCRIPTION" desc="Description for the flag that causes Chrome to use the sandbox (testing) server for Sync.">
         Connects to the testing server for Chrome Sync.
       </message>
+      <if expr="chromeos">
+        <message name="IDS_FLAGS_OZONE_TEST_SINGLE_HARDWARE_OVERLAY" desc="Name of the 'Ozone hardware overlay' lab.">
+          Ozone hardware overlay
+        </message>
+        <message name="IDS_FLAGS_OZONE_TEST_SINGLE_HARDWARE_OVERLAY_DESCRIPTION" desc="Description of the 'Ozone hardware overlay' lab.">
+          Enables compositing with an addition of one hardware overlay when possible.
+        </message>
+      </if>
 
       <!-- Crashes -->
       <message name="IDS_CRASHES_TITLE" desc="Title for the chrome://crashes page.">

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

content/browser/compositor/gpu_process_transport_factory.cc

@@ -50,6 +50,7 @@
 #elif defined(USE_OZONE)
 #include "content/browser/compositor/overlay_candidate_validator_ozone.h"
 #include "content/browser/compositor/software_output_device_ozone.h"
+#include "ui/ozone/public/ozone_switches.h"
 #include "ui/ozone/public/surface_factory_ozone.h"
 #elif defined(USE_X11)
 #include "content/browser/compositor/software_output_device_x11.h"
@@ -121,9 +122,10 @@ scoped_ptr<cc::OverlayCandidateValidator> CreateOverlayCandidateValidator(
 #if defined(USE_OZONE)
   ui::OverlayCandidatesOzone* overlay_candidates =
       ui::SurfaceFactoryOzone::GetInstance()->GetOverlayCandidates(widget);
+  base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
   if (overlay_candidates &&
-      base::CommandLine::ForCurrentProcess()->HasSwitch(
-          switches::kEnableHardwareOverlays)) {
+      (command_line->HasSwitch(switches::kEnableHardwareOverlays) ||
+       command_line->HasSwitch(switches::kOzoneTestSingleOverlaySupport))) {
     return scoped_ptr<cc::OverlayCandidateValidator>(
         new OverlayCandidateValidatorOzone(widget, overlay_candidates));
   }