Reland of cc: Remove frame queuing from the scheduler.

cc/scheduler/scheduler_state_machine.cc

Index: cc/scheduler/scheduler_state_machine.cc
diff --git a/cc/scheduler/scheduler_state_machine.cc b/cc/scheduler/scheduler_state_machine.cc
index 80d45ed2336189eeb0887490b83212cb311e9a38..70c7f67cc66398046fac720c643fcf25ce3c2cc1 100644
--- a/cc/scheduler/scheduler_state_machine.cc
+++ b/cc/scheduler/scheduler_state_machine.cc
@@ -123,10 +123,6 @@ const char* SchedulerStateMachine::BeginMainFrameStateToString(
       return "BEGIN_MAIN_FRAME_STATE_STARTED";
     case BEGIN_MAIN_FRAME_STATE_READY_TO_COMMIT:
       return "BEGIN_MAIN_FRAME_STATE_READY_TO_COMMIT";
-    case BEGIN_MAIN_FRAME_STATE_WAITING_FOR_ACTIVATION:
-      return "BEGIN_MAIN_FRAME_STATE_WAITING_FOR_ACTIVATION";
-    case BEGIN_MAIN_FRAME_STATE_WAITING_FOR_DRAW:
-      return "BEGIN_MAIN_FRAME_STATE_WAITING_FOR_DRAW";
   }
   NOTREACHED();
   return "???";
@@ -367,6 +363,17 @@ bool SchedulerStateMachine::ShouldDraw() const {
   if (begin_impl_frame_state_ != BEGIN_IMPL_FRAME_STATE_INSIDE_DEADLINE)
     return false;
 
+  // Wait for active tree to be rasterized before drawing in browser compositor.
+  if (wait_for_ready_to_draw_) {
+    DCHECK(settings_.commit_to_active_tree);
+    return false;
+  }
+
+  // Browser compositor commit steals any resources submitted in draw. Therefore
+  // drawing while a commit is pending is wasteful.
+  if (settings_.commit_to_active_tree && CommitPending())
+    return false;
+
   // Only handle forced redraws due to timeouts on the regular deadline.
   if (forced_redraw_state_ == FORCED_REDRAW_STATE_WAITING_FOR_DRAW)
     return true;
@@ -429,6 +436,16 @@ bool SchedulerStateMachine::ShouldSendBeginMainFrame() const {
   if (begin_main_frame_state_ != BEGIN_MAIN_FRAME_STATE_IDLE)
     return false;
 
+  // MFBA is disabled and we are waiting for previous activation.
+  if (!settings_.main_frame_before_activation_enabled && has_pending_tree_)
+    return false;
+
+  // We are waiting for previous frame to be drawn, swapped and acked.
+  if (settings_.commit_to_active_tree &&
+      (active_tree_needs_first_draw_ || SwapThrottled())) {
+    return false;
+  }
+
   // Don't send BeginMainFrame early if we are prioritizing the active tree
   // because of ImplLatencyTakesPriority.
   if (ImplLatencyTakesPriority() &&
@@ -484,10 +501,13 @@ bool SchedulerStateMachine::ShouldCommit() const {
     return false;
   }
 
-  // If we only have an active tree, it is incorrect to replace it
-  // before we've drawn it.
+  // If we only have an active tree, it is incorrect to replace it before we've
+  // drawn it.
   DCHECK(!settings_.commit_to_active_tree || !active_tree_needs_first_draw_);
 
+  // In browser compositor commit reclaims any resources submitted during draw.
+  DCHECK(!settings_.commit_to_active_tree || !SwapThrottled());
+
   return true;
 }
 
@@ -566,12 +586,7 @@ void SchedulerStateMachine::WillCommit(bool commit_has_no_updates) {
           commit_has_no_updates));
   commit_count_++;
   last_commit_had_no_updates_ = commit_has_no_updates;
-
-  if (commit_has_no_updates || settings_.main_frame_before_activation_enabled) {
-    begin_main_frame_state_ = BEGIN_MAIN_FRAME_STATE_IDLE;
-  } else {
-    begin_main_frame_state_ = BEGIN_MAIN_FRAME_STATE_WAITING_FOR_ACTIVATION;
-  }
+  begin_main_frame_state_ = BEGIN_MAIN_FRAME_STATE_IDLE;
 
   if (!commit_has_no_updates) {
     // Pending tree only exists if commit had updates.
@@ -597,13 +612,6 @@ void SchedulerStateMachine::WillCommit(bool commit_has_no_updates) {
 }
 
 void SchedulerStateMachine::WillActivate() {
-  if (begin_main_frame_state_ ==
-      BEGIN_MAIN_FRAME_STATE_WAITING_FOR_ACTIVATION) {
-    begin_main_frame_state_ = settings_.commit_to_active_tree
-                                  ? BEGIN_MAIN_FRAME_STATE_WAITING_FOR_DRAW
-                                  : BEGIN_MAIN_FRAME_STATE_IDLE;
-  }
-
   if (compositor_frame_sink_state_ ==
       COMPOSITOR_FRAME_SINK_WAITING_FOR_FIRST_ACTIVATION)
     compositor_frame_sink_state_ = COMPOSITOR_FRAME_SINK_ACTIVE;
@@ -636,9 +644,6 @@ void SchedulerStateMachine::WillDrawInternal() {
 
   if (forced_redraw_state_ == FORCED_REDRAW_STATE_WAITING_FOR_DRAW)
     forced_redraw_state_ = FORCED_REDRAW_STATE_IDLE;
-
-  if (begin_main_frame_state_ == BEGIN_MAIN_FRAME_STATE_WAITING_FOR_DRAW)
-    begin_main_frame_state_ = BEGIN_MAIN_FRAME_STATE_IDLE;
 }
 
 void SchedulerStateMachine::DidDrawInternal(DrawResult draw_result) {
@@ -871,12 +876,12 @@ SchedulerStateMachine::CurrentBeginImplFrameDeadlineMode() const {
   if (settings_.using_synchronous_renderer_compositor) {
     // No deadline for synchronous compositor.
     return BEGIN_IMPL_FRAME_DEADLINE_MODE_NONE;
-  } else if (ShouldTriggerBeginImplFrameDeadlineImmediately()) {
-    return BEGIN_IMPL_FRAME_DEADLINE_MODE_IMMEDIATE;
   } else if (wait_for_ready_to_draw_) {
-    // When we are waiting for ready to draw signal, we do not wait to post a
-    // deadline yet.
+    // In browser compositor, wait for active tree to be rasterized.
+    DCHECK(settings_.commit_to_active_tree);
     return BEGIN_IMPL_FRAME_DEADLINE_MODE_BLOCKED_ON_READY_TO_DRAW;
+  } else if (ShouldTriggerBeginImplFrameDeadlineImmediately()) {
+    return BEGIN_IMPL_FRAME_DEADLINE_MODE_IMMEDIATE;
   } else if (needs_redraw_) {
     // We have an animation or fast input path on the impl thread that wants
     // to draw, so don't wait too long for a new active tree.
@@ -894,11 +899,6 @@ bool SchedulerStateMachine::ShouldTriggerBeginImplFrameDeadlineImmediately()
   if (PendingActivationsShouldBeForced() && !has_pending_tree_)
     return true;
 
-  // Do not trigger deadline immediately if we're waiting for READY_TO_DRAW
-  // unless it's one of the forced cases.
-  if (wait_for_ready_to_draw_)
-    return false;
-
   // SwapAck throttle the deadline since we wont draw and swap anyway.
   if (SwapThrottled())
     return false;
@@ -914,8 +914,7 @@ bool SchedulerStateMachine::ShouldTriggerBeginImplFrameDeadlineImmediately()
   // don't have a pending tree -- otherwise we should give it a chance to
   // activate.
   // TODO(skyostil): Revisit this when we have more accurate deadline estimates.
-  if (begin_main_frame_state_ == BEGIN_MAIN_FRAME_STATE_IDLE &&
-      !has_pending_tree_)
+  if (!CommitPending() && !has_pending_tree_)
     return true;
 
   // Prioritize impl-thread draws in ImplLatencyTakesPriority mode.
@@ -1032,9 +1031,8 @@ void SchedulerStateMachine::NotifyReadyToCommit() {
   DCHECK_EQ(begin_main_frame_state_, BEGIN_MAIN_FRAME_STATE_STARTED)
       << AsValue()->ToString();
   begin_main_frame_state_ = BEGIN_MAIN_FRAME_STATE_READY_TO_COMMIT;
-  // In commit_to_active_tree mode, commit should happen right after
-  // BeginFrame, meaning when this function is called, next action should be
-  // commit.
+  // In commit_to_active_tree mode, commit should happen right after BeginFrame,
+  // meaning when this function is called, next action should be commit.
   if (settings_.commit_to_active_tree)
     DCHECK(ShouldCommit());
 }