Read compositor VSync information from platform, when possible

ui/compositor/compositor_vsync_manager.cc

Index: ui/compositor/compositor_vsync_manager.cc
diff --git a/ui/compositor/compositor_vsync_manager.cc b/ui/compositor/compositor_vsync_manager.cc
new file mode 100644
index 0000000000000000000000000000000000000000..f6fe895fb433ef7eaa88048eec6eed5934beb79c
--- /dev/null
+++ b/ui/compositor/compositor_vsync_manager.cc
@@ -0,0 +1,63 @@
+// Copyright (c) 2014 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 "ui/compositor/compositor_vsync_manager.h"
+
+namespace ui {
+
+CompositorVSyncManager::CompositorVSyncManager()
+    : observer_list_(new ObserverListThreadSafe<Observer>()),
+      authoritative_vsync_interval_(base::TimeDelta::FromSeconds(0)) {}
+
+CompositorVSyncManager::~CompositorVSyncManager() {}
+
+void CompositorVSyncManager::SetAuthoritativeVSyncInterval(
+    base::TimeDelta interval) {
+  base::TimeTicks timebase;
+  {
+    base::AutoLock lock(vsync_parameters_lock_);
+    timebase = last_timebase_;
+    authoritative_vsync_interval_ = interval;
+    last_interval_ = interval;
+  }
+  NotifyObservers(timebase, interval);
+}
+
+void CompositorVSyncManager::UpdateVSyncParameters(base::TimeTicks timebase,
+                                                   base::TimeDelta interval) {
+  {
+    base::AutoLock lock(vsync_parameters_lock_);
+    if (authoritative_vsync_interval_ != base::TimeDelta::FromSeconds(0))
+      interval = authoritative_vsync_interval_;
+    last_timebase_ = timebase;
+    last_interval_ = interval;
+  }
+  NotifyObservers(timebase, interval);
+}
+
+void CompositorVSyncManager::AddObserver(Observer* observer) {
+  base::TimeTicks timebase;
+  base::TimeDelta interval;
+  {
+    base::AutoLock lock(vsync_parameters_lock_);
+    timebase = last_timebase_;
+    interval = last_interval_;
+  }
+  observer_list_->AddObserver(observer);
+  observer->OnUpdateVSyncParameters(timebase, interval);
+}
+
+void CompositorVSyncManager::RemoveObserver(Observer* observer) {
+  observer_list_->RemoveObserver(observer);
+}
+
+void CompositorVSyncManager::NotifyObservers(base::TimeTicks timebase,
+                                             base::TimeDelta interval) {
+  observer_list_->Notify(
+      &CompositorVSyncManager::Observer::OnUpdateVSyncParameters,
+      timebase,
+      interval);
+}
+
+}  // namespace ui