[exp] cc: Introduce cc::CompositorMutator.

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 189387cf12856f1fc348c605a976d1422aaf705c..ca943f3c0af7be3ce90174b0f835d989de3884e9 100644
--- a/cc/scheduler/scheduler_state_machine.cc
+++ b/cc/scheduler/scheduler_state_machine.cc
@@ -23,6 +23,7 @@ SchedulerStateMachine::SchedulerStateMachine(const SchedulerSettings& settings)
       commit_count_(0),
       current_frame_number_(0),
       last_frame_number_animate_performed_(-1),
+      last_frame_number_custom_mutate_performed_(-1),
       last_frame_number_swap_performed_(-1),
       last_frame_number_swap_requested_(-1),
       last_frame_number_begin_main_frame_sent_(-1),
@@ -126,6 +127,8 @@ const char* SchedulerStateMachine::ActionToString(Action action) {
       return "ACTION_NONE";
     case ACTION_ANIMATE:
       return "ACTION_ANIMATE";
+    case ACTION_CUSTOM_MUTATE:
+      return "ACTION_CUSTOM_MUTATE";
     case ACTION_SEND_BEGIN_MAIN_FRAME:
       return "ACTION_SEND_BEGIN_MAIN_FRAME";
     case ACTION_COMMIT:
@@ -199,6 +202,8 @@ void SchedulerStateMachine::AsValueInto(base::trace_event::TracedValue* state,
 
   state->SetInteger("last_frame_number_animate_performed",
                     last_frame_number_animate_performed_);
+  state->SetInteger("last_frame_number_custom_mutate_performed",
+                    last_frame_number_custom_mutate_performed_);
   state->SetInteger("last_frame_number_swap_performed",
                     last_frame_number_swap_performed_);
   state->SetInteger("last_frame_number_swap_requested",
@@ -256,6 +261,10 @@ bool SchedulerStateMachine::HasAnimatedThisFrame() const {
   return last_frame_number_animate_performed_ == current_frame_number_;
 }
 
+bool SchedulerStateMachine::HasCustomMutatedThisFrame() const {
+  return last_frame_number_custom_mutate_performed_ == current_frame_number_;
+}
+
 bool SchedulerStateMachine::HasSentBeginMainFrameThisFrame() const {
   return current_frame_number_ ==
          last_frame_number_begin_main_frame_sent_;
@@ -405,6 +414,21 @@ bool SchedulerStateMachine::ShouldAnimate() const {
   return needs_redraw_ || needs_animate_;
 }
 
+bool SchedulerStateMachine::ShouldCustomMutate() const {
+  // Don't mutate if we are waiting on the first commit after a surface.
+  if (output_surface_state_ != OUTPUT_SURFACE_ACTIVE)
+    return false;
+
+  if (HasCustomMutatedThisFrame() || !needs_custom_mutate_)
+    return false;
+
+  if (begin_impl_frame_state_ != BEGIN_IMPL_FRAME_STATE_BEGIN_FRAME_STARTING &&
+      begin_impl_frame_state_ != BEGIN_IMPL_FRAME_STATE_INSIDE_DEADLINE)
+    return false;
+
+  return true;
+}
+
 bool SchedulerStateMachine::CouldSendBeginMainFrame() const {
   if (!needs_commit_)
     return false;
@@ -511,6 +535,8 @@ SchedulerStateMachine::Action SchedulerStateMachine::NextAction() const {
     return ACTION_COMMIT;
   if (ShouldAnimate())
     return ACTION_ANIMATE;
+  if (ShouldCustomMutate())
+    return ACTION_CUSTOM_MUTATE;
   if (ShouldDraw()) {
     if (PendingDrawsShouldBeAborted())
       return ACTION_DRAW_AND_SWAP_ABORT;
@@ -546,6 +572,11 @@ void SchedulerStateMachine::UpdateState(Action action) {
       SetNeedsRedraw();
       return;
 
+    case ACTION_CUSTOM_MUTATE:
+      last_frame_number_custom_mutate_performed_ = current_frame_number_;
+      needs_custom_mutate_ = false;
+      return;
+
     case ACTION_SEND_BEGIN_MAIN_FRAME:
       DCHECK(!has_pending_tree_ ||
              settings_.main_frame_before_activation_enabled);
@@ -962,6 +993,10 @@ void SchedulerStateMachine::SetNeedsAnimate() {
   needs_animate_ = true;
 }
 
+void SchedulerStateMachine::SetNeedsCustomMutate() {
+  needs_custom_mutate_ = true;
+}
+
 void SchedulerStateMachine::SetNeedsPrepareTiles() {
   if (!needs_prepare_tiles_) {
     TRACE_EVENT0("cc", "SchedulerStateMachine::SetNeedsPrepareTiles");