cc: Remove retro frame logic from scheduler.

cc/scheduler/scheduler.cc

Index: cc/scheduler/scheduler.cc
diff --git a/cc/scheduler/scheduler.cc b/cc/scheduler/scheduler.cc
index 4fa133cb3538e608f32053b216d68aef12fdcfc4..8c744074c785e219434ebb68da0d155fbea20c4f 100644
--- a/cc/scheduler/scheduler.cc
+++ b/cc/scheduler/scheduler.cc
@@ -64,8 +64,6 @@ Scheduler::Scheduler(
   DCHECK(client_);
   DCHECK(!state_machine_.BeginFrameNeeded());
 
-  begin_retro_frame_closure_ =
-      base::Bind(&Scheduler::BeginRetroFrame, weak_factory_.GetWeakPtr());
   begin_impl_frame_deadline_closure_ = base::Bind(
       &Scheduler::OnBeginImplFrameDeadline, weak_factory_.GetWeakPtr());
 
@@ -200,8 +198,6 @@ void Scheduler::DidPrepareTiles() {
 
 void Scheduler::DidLoseOutputSurface() {
   TRACE_EVENT0("cc", "Scheduler::DidLoseOutputSurface");
-  begin_retro_frame_args_.clear();
-  begin_retro_frame_task_.Cancel();
   state_machine_.DidLoseOutputSurface();
   UpdateCompositorTimingHistoryRecordingEnabled();
   ProcessScheduledActions();
@@ -251,6 +247,7 @@ void Scheduler::SetupNextBeginFrameIfNeeded() {
     } else if (state_machine_.begin_impl_frame_state() ==
                SchedulerStateMachine::BEGIN_IMPL_FRAME_STATE_IDLE) {
       // Call RemoveObserver in between frames only.
+      missed_begin_frame_task_.Cancel();
       observing_begin_frame_source_ = false;
       if (begin_frame_source_)
         begin_frame_source_->RemoveObserver(this);
@@ -259,8 +256,6 @@ void Scheduler::SetupNextBeginFrameIfNeeded() {
                                                        false);
     }
   }
-
-  PostBeginRetroFrameIfNeeded();
 }
 
 void Scheduler::OnBeginFrameSourcePausedChanged(bool paused) {
@@ -274,11 +269,21 @@ void Scheduler::OnBeginFrameSourcePausedChanged(bool paused) {
 
 // BeginFrame is the mechanism that tells us that now is a good time to start
 // making a frame. Usually this means that user input for the frame is complete.
-// If the scheduler is busy, we queue the BeginFrame to be handled later as
-// a BeginRetroFrame.
+// If the scheduler is busy, we finish the previous frame before starting a new
+// one.
 bool Scheduler::OnBeginFrameDerivedImpl(const BeginFrameArgs& args) {
   TRACE_EVENT1("cc,benchmark", "Scheduler::BeginFrame", "args", args.AsValue());
 
+  if (state_machine_.begin_impl_frame_state() !=
+      SchedulerStateMachine::BEGIN_IMPL_FRAME_STATE_IDLE) {
+    OnBeginImplFrameDeadline();
+  }
+
+  if (!observing_begin_frame_source_) {
+    DCHECK(!state_machine_.BeginFrameNeeded());
+    return false;
+  }
+
   // Trace this begin frame time through the Chrome stack
   TRACE_EVENT_FLOW_BEGIN0(
       TRACE_DISABLED_BY_DEFAULT("cc.debug.scheduler.frames"), "BeginFrameArgs",
@@ -290,31 +295,12 @@ bool Scheduler::OnBeginFrameDerivedImpl(const BeginFrameArgs& args) {
 
   if (settings_.using_synchronous_renderer_compositor) {
     BeginImplFrameSynchronous(adjusted_args);
-    return true;
-  }
-
-  // We have just called SetNeedsBeginFrame(true) and the BeginFrameSource has
-  // sent us the last BeginFrame we have missed. As we might not be able to
-  // actually make rendering for this call, handle it like a "retro frame".
-  // TODO(brainderson): Add a test for this functionality ASAP!
-  if (adjusted_args.type == BeginFrameArgs::MISSED) {
-    begin_retro_frame_args_.push_back(adjusted_args);
-    PostBeginRetroFrameIfNeeded();
-    return true;
-  }
-
-  bool should_defer_begin_frame =
-      !begin_retro_frame_args_.empty() ||
-      !begin_retro_frame_task_.IsCancelled() ||
-      !observing_begin_frame_source_ ||
-      (state_machine_.begin_impl_frame_state() !=
-       SchedulerStateMachine::BEGIN_IMPL_FRAME_STATE_IDLE);
-
-  if (should_defer_begin_frame) {
-    begin_retro_frame_args_.push_back(adjusted_args);
-    TRACE_EVENT_INSTANT0("cc", "Scheduler::BeginFrame deferred",
-                         TRACE_EVENT_SCOPE_THREAD);
-    // Queuing the frame counts as "using it", so we need to return true.
+  } else if (args.type == BeginFrameArgs::MISSED) {
+    DCHECK(missed_begin_frame_task_.IsCancelled());
+    missed_begin_frame_task_.Reset(
+        base::Bind(&Scheduler::BeginImplFrameWithDeadline,
+                   weak_factory_.GetWeakPtr(), adjusted_args));
+    task_runner_->PostTask(FROM_HERE, missed_begin_frame_task_.callback());
   } else {
     BeginImplFrameWithDeadline(adjusted_args);
   }
@@ -341,78 +327,9 @@ void Scheduler::OnDrawForOutputSurface(bool resourceless_software_draw) {
   state_machine_.SetResourcelessSoftwareDraw(false);
 }
 
-// BeginRetroFrame is called for BeginFrames that we've deferred because
-// the scheduler was in the middle of processing a previous BeginFrame.
-void Scheduler::BeginRetroFrame() {
-  TRACE_EVENT0("cc,benchmark", "Scheduler::BeginRetroFrame");
-  DCHECK(!settings_.using_synchronous_renderer_compositor);
-  DCHECK(!begin_retro_frame_args_.empty());
-  DCHECK(!begin_retro_frame_task_.IsCancelled());
-  DCHECK_EQ(state_machine_.begin_impl_frame_state(),
-            SchedulerStateMachine::BEGIN_IMPL_FRAME_STATE_IDLE);
-
-  begin_retro_frame_task_.Cancel();
-
-  // Discard expired BeginRetroFrames
-  // Today, we should always end up with at most one un-expired BeginRetroFrame
-  // because deadlines will not be greater than the next frame time. We don't
-  // DCHECK though because some systems don't always have monotonic timestamps.
-  // TODO(brianderson): In the future, long deadlines could result in us not
-  // draining the queue if we don't catch up. If we consistently can't catch
-  // up, our fallback should be to lower our frame rate.
-  base::TimeTicks now = Now();
-
-  while (!begin_retro_frame_args_.empty()) {
-    const BeginFrameArgs& args = begin_retro_frame_args_.front();
-    base::TimeTicks expiration_time = args.deadline;
-    if (now <= expiration_time)
-      break;
-    TRACE_EVENT_INSTANT2(
-        "cc", "Scheduler::BeginRetroFrame discarding", TRACE_EVENT_SCOPE_THREAD,
-        "expiration_time - now", (expiration_time - now).InMillisecondsF(),
-        "BeginFrameArgs", begin_retro_frame_args_.front().AsValue());
-    begin_retro_frame_args_.pop_front();
-    if (begin_frame_source_)
-      begin_frame_source_->DidFinishFrame(this, begin_retro_frame_args_.size());
-  }
-
-  if (begin_retro_frame_args_.empty()) {
-    TRACE_EVENT_INSTANT0("cc", "Scheduler::BeginRetroFrames all expired",
-                         TRACE_EVENT_SCOPE_THREAD);
-  } else {
-    BeginFrameArgs front = begin_retro_frame_args_.front();
-    begin_retro_frame_args_.pop_front();
-    BeginImplFrameWithDeadline(front);
-  }
-}
-
-// There could be a race between the posted BeginRetroFrame and a new
-// BeginFrame arriving via the normal mechanism. Scheduler::BeginFrame
-// will check if there is a pending BeginRetroFrame to ensure we handle
-// BeginFrames in FIFO order.
-void Scheduler::PostBeginRetroFrameIfNeeded() {
-  TRACE_EVENT1(TRACE_DISABLED_BY_DEFAULT("cc.debug.scheduler"),
-               "Scheduler::PostBeginRetroFrameIfNeeded", "state", AsValue());
-  if (!observing_begin_frame_source_)
-    return;
-
-  if (begin_retro_frame_args_.empty() || !begin_retro_frame_task_.IsCancelled())
-    return;
-
-  // begin_retro_frame_args_ should always be empty for the
-  // synchronous compositor.
-  DCHECK(!settings_.using_synchronous_renderer_compositor);
-
-  if (state_machine_.begin_impl_frame_state() !=
-      SchedulerStateMachine::BEGIN_IMPL_FRAME_STATE_IDLE)
-    return;
-
-  begin_retro_frame_task_.Reset(begin_retro_frame_closure_);
-
-  task_runner_->PostTask(FROM_HERE, begin_retro_frame_task_.callback());
-}
-
 void Scheduler::BeginImplFrameWithDeadline(const BeginFrameArgs& args) {
+  missed_begin_frame_task_.Cancel();
+
   bool main_thread_is_in_high_latency_mode =
       state_machine_.main_thread_missed_last_deadline();
   TRACE_EVENT2("cc,benchmark", "Scheduler::BeginImplFrame", "args",
@@ -464,7 +381,7 @@ void Scheduler::BeginImplFrameWithDeadline(const BeginFrameArgs& args) {
     TRACE_EVENT_INSTANT0("cc", "SkipBeginImplFrameToReduceLatency",
                          TRACE_EVENT_SCOPE_THREAD);
     if (begin_frame_source_)
-      begin_frame_source_->DidFinishFrame(this, begin_retro_frame_args_.size());
+      begin_frame_source_->DidFinishFrame(this, 0);
     return;
   }
 
@@ -493,7 +410,7 @@ void Scheduler::FinishImplFrame() {
 
   client_->DidFinishImplFrame();
   if (begin_frame_source_)
-    begin_frame_source_->DidFinishFrame(this, begin_retro_frame_args_.size());
+    begin_frame_source_->DidFinishFrame(this, 0);
   begin_impl_frame_tracker_.Finish();
 }
 
@@ -738,10 +655,6 @@ Scheduler::AsValue() const {
                    estimated_parent_draw_time_.InMillisecondsF());
   state->SetBoolean("observing_begin_frame_source",
                     observing_begin_frame_source_);
-  state->SetInteger("begin_retro_frame_args",
-                    static_cast<int>(begin_retro_frame_args_.size()));
-  state->SetBoolean("begin_retro_frame_task",
-                    !begin_retro_frame_task_.IsCancelled());
   state->SetBoolean("begin_impl_frame_deadline_task",
                     !begin_impl_frame_deadline_task_.IsCancelled());
   state->SetString("inside_action",