ozone: Allow ozone evdev/kvm features to be enabled without running on ChromeOS.

chromeos/system/devicemode.cc

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chromeos/system/devicemode.h"
+
+#include "base/command_line.h"
+#include "base/lazy_instance.h"
+#include "base/sys_info.h"
+#include "chromeos/chromeos_switches.h"
+
+namespace chromeos {
+namespace {
+
+class ChromeOSDeviceMode {
+ public:
+  ChromeOSDeviceMode() {
+    // Running as system compositor by default when running on ChromeOS.
+    is_running_as_system_compositor_ = base::SysInfo::IsRunningOnChromeOS();
+
+    base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
+    if (command_line->HasSwitch(switches::kEnableSystemCompositorMode))
+      is_running_as_system_compositor_ = true;
+    if (command_line->HasSwitch(switches::kDisableSystemCompositorMode))
+      is_running_as_system_compositor_ = false;
+  }
+
+  bool is_running_as_system_compositor() const {
+    return is_running_as_system_compositor_;
+  }
+
+ private:
+  bool is_running_as_system_compositor_;
+};
+
+base::LazyInstance<ChromeOSDeviceMode>::Leaky g_chrome_os_device_mode =
+    LAZY_INSTANCE_INITIALIZER;

[oshima, 2017/01/05 22:43:57]

Do you need lazy instance?

Since this never change during runtime, a local static inside
IsRunningAsSystemComopsitor is sufficient?

[reveman, 2017/01/06 08:15:14]

Done. I assume it's OK to rely on the compiler's thread-safe initialization of
statics as you suggested this. Never clear to me if that was OK or not in
chromium code. We better not compile with --fno-threadsafe-statics.. 

[oshima, 2017/01/09 19:09:12]

CommandLine isn't thread safe, so the original code is already thread unsafe,
although it shouldn't be a real issue because it won't be modified once
initialized, and it'll be always initialized to the same value.

[reveman, 2017/01/09 22:51:50]

CommandLine::HasSwitch is const so should be thread-safe unless Chromium is
actually modifying the command line after startup, no? Either way, this is
currently only used from the main thread so shouldn't be an issue in practice.

+
+}  // namespace
+
+bool IsRunningAsSystemCompositor() {
+  return g_chrome_os_device_mode.Get().is_running_as_system_compositor();
+}
+
+}  // namespace chromeos