cc::SurfaceDependencyTracker should not crash when a Display goes away

cc/surfaces/primary_begin_frame_source.cc

Index: cc/surfaces/primary_begin_frame_source.cc
diff --git a/cc/surfaces/primary_begin_frame_source.cc b/cc/surfaces/primary_begin_frame_source.cc
new file mode 100644
index 0000000000000000000000000000000000000000..588dbfae67e07ba6d65c40d3376d75e14e2d37ac
--- /dev/null
+++ b/cc/surfaces/primary_begin_frame_source.cc
@@ -0,0 +1,87 @@
+// 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 "cc/surfaces/primary_begin_frame_source.h"
+
+namespace cc {
+
+PrimaryBeginFrameSource::PrimaryBeginFrameSource()
+    : begin_frame_source_(this) {}
+
+PrimaryBeginFrameSource::~PrimaryBeginFrameSource() = default;
+
+void PrimaryBeginFrameSource::OnBeginFrameSourceAdded(
+    BeginFrameSource* begin_frame_source) {
+  sources_.insert(begin_frame_source);
+
+  if (current_begin_frame_source_)
+    return;
+
+  current_begin_frame_source_ = begin_frame_source;
+  if (needs_begin_frames_ && current_begin_frame_source_)
+    current_begin_frame_source_->AddObserver(this);
+}
+
+void PrimaryBeginFrameSource::OnBeginFrameSourceRemoved(
+    BeginFrameSource* begin_frame_source) {
+  sources_.erase(begin_frame_source);
+  if (current_begin_frame_source_ != begin_frame_source)
+    return;
+
+  if (needs_begin_frames_)
+    current_begin_frame_source_->RemoveObserver(this);
+
+  if (!sources_.empty())
+    current_begin_frame_source_ = *sources_.begin();
+  else
+    current_begin_frame_source_ = nullptr;
+
+  if (needs_begin_frames_ && current_begin_frame_source_)
+    current_begin_frame_source_->AddObserver(this);
+}
+
+void PrimaryBeginFrameSource::OnBeginFrame(const BeginFrameArgs& args) {
+  begin_frame_source_.OnBeginFrame(args);
+  last_begin_frame_args_ = args;
+}
+
+const BeginFrameArgs& PrimaryBeginFrameSource::LastUsedBeginFrameArgs() const {
+  return last_begin_frame_args_;
+}
+
+void PrimaryBeginFrameSource::OnBeginFrameSourcePausedChanged(bool paused) {}
+
+void PrimaryBeginFrameSource::DidFinishFrame(BeginFrameObserver* obs,
+                                             const BeginFrameAck& ack) {
+  begin_frame_source_.DidFinishFrame(obs, ack);
+}
+
+void PrimaryBeginFrameSource::AddObserver(BeginFrameObserver* obs) {
+  begin_frame_source_.AddObserver(obs);
+}
+
+void PrimaryBeginFrameSource::RemoveObserver(BeginFrameObserver* obs) {
+  begin_frame_source_.RemoveObserver(obs);
+}
+
+bool PrimaryBeginFrameSource::IsThrottled() const {
+  return begin_frame_source_.IsThrottled();
+}
+
+void PrimaryBeginFrameSource::OnNeedsBeginFrames(bool needs_begin_frames) {
+  if (needs_begin_frames_ == needs_begin_frames)
+    return;
+
+  needs_begin_frames_ = needs_begin_frames;
+
+  if (!current_begin_frame_source_)
+    return;
+
+  if (needs_begin_frames_)
+    current_begin_frame_source_->AddObserver(this);
+  else
+    current_begin_frame_source_->RemoveObserver(this);
+}
+
+}  // namespace cc