Make ui::Compositor BeginFrame subscription robust

ui/compositor/compositor.cc

Index: ui/compositor/compositor.cc
diff --git a/ui/compositor/compositor.cc b/ui/compositor/compositor.cc
index cab5d6eab3787386d62b9a60c15ced8ef41c4663..767595caa95982dfc377c9107a2c84888084cd6b 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -190,8 +190,6 @@ Compositor::~Compositor() {
   FOR_EACH_OBSERVER(CompositorAnimationObserver, animation_observer_list_,
                     OnCompositingShuttingDown(this));
 
-  DCHECK(begin_frame_observer_list_.empty());
-
   if (root_layer_)
     root_layer_->ResetCompositor();
 
@@ -355,27 +353,20 @@ bool Compositor::HasAnimationObserver(
 }
 
 void Compositor::AddBeginFrameObserver(CompositorBeginFrameObserver* observer) {
-  DCHECK(std::find(begin_frame_observer_list_.begin(),
-                   begin_frame_observer_list_.end(), observer) ==
-         begin_frame_observer_list_.end());
-
-  if (begin_frame_observer_list_.empty())
+  if (!begin_frame_observer_list_.might_have_observers())
     host_->SetChildrenNeedBeginFrames(true);
 
+  begin_frame_observer_list_.AddObserver(observer);
+
   if (missed_begin_frame_args_.IsValid())
     observer->OnSendBeginFrame(missed_begin_frame_args_);
-
-  begin_frame_observer_list_.push_back(observer);
 }
 
 void Compositor::RemoveBeginFrameObserver(
     CompositorBeginFrameObserver* observer) {
-  auto it = std::find(begin_frame_observer_list_.begin(),
-                      begin_frame_observer_list_.end(), observer);
-  DCHECK(it != begin_frame_observer_list_.end());
-  begin_frame_observer_list_.erase(it);
+  begin_frame_observer_list_.RemoveObserver(observer);
 
-  if (begin_frame_observer_list_.empty()) {
+  if (!begin_frame_observer_list_.might_have_observers()) {
     host_->SetChildrenNeedBeginFrames(false);

[danakj, 2015/10/21 20:23:51]

What if might_have is true, but actually this was the last observer and you
don't have any now. Where will we set false on the host?

[jdduke (slow), 2015/10/21 20:48:12]

Good call, we can check after iterating over the observers in
SendBeginFramesToChildren.

     missed_begin_frame_args_ = cc::BeginFrameArgs();
   }
@@ -448,8 +439,8 @@ void Compositor::DidAbortSwapBuffers() {
 }
 
 void Compositor::SendBeginFramesToChildren(const cc::BeginFrameArgs& args) {
-  for (auto observer : begin_frame_observer_list_)
-    observer->OnSendBeginFrame(args);
+  FOR_EACH_OBSERVER(CompositorBeginFrameObserver, begin_frame_observer_list_,
+                    OnSendBeginFrame(args));
 
   missed_begin_frame_args_ = args;
   missed_begin_frame_args_.type = cc::BeginFrameArgs::MISSED;