[cc] Set BeginFrame sequence numbers on CompositorFrames from Scheduler.

cc/scheduler/scheduler.cc

Index: cc/scheduler/scheduler.cc
diff --git a/cc/scheduler/scheduler.cc b/cc/scheduler/scheduler.cc
index 66de03d3139dd6c003e698e4ecd5dce33c106308..93525696d74a3c0e0b7b79e14824ac9bb150e6a7 100644
--- a/cc/scheduler/scheduler.cc
+++ b/cc/scheduler/scheduler.cc
@@ -798,4 +798,11 @@ bool Scheduler::IsBeginMainFrameSentOrStarted() const {
               SchedulerStateMachine::BEGIN_MAIN_FRAME_STATE_STARTED);
 }
 
+BeginFrameAck Scheduler::CurrentBeginFrameAckForActiveTree() const {
+  return BeginFrameAck(
+      begin_main_frame_args_.source_id, begin_main_frame_args_.sequence_number,
+      state_machine_.last_begin_frame_sequence_number_active_tree_was_fresh(),
+      0, true);
+}
+
 }  // namespace cc