Allow cc::Scheduler begin frame source to be changed

cc/scheduler/scheduler.cc

Index: cc/scheduler/scheduler.cc
diff --git a/cc/scheduler/scheduler.cc b/cc/scheduler/scheduler.cc
index 3f5c3a39e3a429680375f6cb665bff39dc519c0f..394986ec55981df2a1191e32820b4b984751fdd7 100644
--- a/cc/scheduler/scheduler.cc
+++ b/cc/scheduler/scheduler.cc
@@ -49,7 +49,7 @@ Scheduler::Scheduler(
       client_(client),
       layer_tree_host_id_(layer_tree_host_id),
       task_runner_(task_runner),
-      begin_frame_source_(begin_frame_source),
+      begin_frame_source_(nullptr),
       observing_begin_frame_source_(false),
       compositor_timing_history_(std::move(compositor_timing_history)),
       begin_impl_frame_deadline_mode_(
@@ -68,7 +68,7 @@ Scheduler::Scheduler(
   begin_impl_frame_deadline_closure_ = base::Bind(
       &Scheduler::OnBeginImplFrameDeadline, weak_factory_.GetWeakPtr());
 
-  begin_frame_source_->SetClientReady();
+  SetBeginFrameSource(begin_frame_source);
   ProcessScheduledActions();
 }
 
@@ -114,6 +114,16 @@ void Scheduler::NotifyReadyToDraw() {
   ProcessScheduledActions();
 }
 
+void Scheduler::SetBeginFrameSource(BeginFrameSource* source) {

[Sami, 2016/03/09 17:09:15]

Not sure how often this would actually be used, but is it worth adding a check
for trying to switch to the same |source| we already have? I think
Add/RemoveObserver might involve IPC in some cases.

[enne (OOO), 2016/03/09 19:46:10]

Yeah, I think something needs to change here, as the scheduler has no idea
whether or not a source has had that called on it before.  I'll put up a patch
to address that separately.

+  DCHECK(source);
+  if (begin_frame_source_ && observing_begin_frame_source_)
+    begin_frame_source_->RemoveObserver(this);
+  begin_frame_source_ = source;
+  if (observing_begin_frame_source_)
+    begin_frame_source_->AddObserver(this);
+  begin_frame_source_->SetClientReady();

[Sami, 2016/03/09 17:09:15]

One problem here is that some begin frame sources (e.g., 
CompositorExternalBeginFrameSource) don't like it if you call SetClientReady
multiple times. Maybe we should change the API contract to allow that. Or maybe
SetClientReady could be folded into AddObserver.

+}
+
 void Scheduler::SetNeedsBeginMainFrame() {
   state_machine_.SetNeedsBeginMainFrame();
   ProcessScheduledActions();
@@ -745,7 +755,7 @@ void Scheduler::AsValueInto(base::trace_event::TracedValue* state) const {
   TRACE_EVENT_CATEGORY_GROUP_ENABLED(
       TRACE_DISABLED_BY_DEFAULT("cc.debug.scheduler.frames"),
       &frame_tracing_enabled);
-  if (frame_tracing_enabled) {
+  if (frame_tracing_enabled && begin_frame_source_) {
     state->BeginDictionary("begin_frame_source_");
     begin_frame_source_->AsValueInto(state);
     state->EndDictionary();