GPU: Only allow the UI channel to preempt if all stubs are scheduled.

gpu/command_buffer/service/gpu_scheduler.cc

Index: gpu/command_buffer/service/gpu_scheduler.cc
diff --git a/gpu/command_buffer/service/gpu_scheduler.cc b/gpu/command_buffer/service/gpu_scheduler.cc
index 78739dca5fcf9b0899c97e2aeb15b59ec0cdc6bd..61bdcf8470e652631741885edcef724f8f94fb81 100644
--- a/gpu/command_buffer/service/gpu_scheduler.cc
+++ b/gpu/command_buffer/service/gpu_scheduler.cc
@@ -136,11 +136,12 @@ void GpuScheduler::SetScheduled(bool scheduled) {
       // state, cancel the task that would reschedule it after a timeout.
       reschedule_task_factory_.InvalidateWeakPtrs();
 
-      if (!scheduled_callback_.is_null())
-        scheduled_callback_.Run();
+      if (!scheduling_changed_callback_.is_null())
+        scheduling_changed_callback_.Run(true);
     }
   } else {
-    if (unscheduled_count_ == 0) {
+    ++unscheduled_count_;
+    if (unscheduled_count_ == 1) {
       TRACE_EVENT_ASYNC_BEGIN1("gpu", "ProcessingSwap", this,
                                "GpuScheduler", this);
 #if defined(OS_WIN)
@@ -155,9 +156,9 @@ void GpuScheduler::SetScheduled(bool scheduled) {
             base::TimeDelta::FromMilliseconds(kRescheduleTimeOutDelay));
       }
 #endif
+      if (!scheduling_changed_callback_.is_null())
+        scheduling_changed_callback_.Run(false);
     }
-
-    ++unscheduled_count_;
   }
 }
 
@@ -170,9 +171,9 @@ bool GpuScheduler::HasMoreWork() {
          (decoder_ && decoder_->ProcessPendingQueries());
 }
 
-void GpuScheduler::SetScheduledCallback(
-    const base::Closure& scheduled_callback) {
-  scheduled_callback_ = scheduled_callback;
+void GpuScheduler::SetSchedulingChangedCallback(
+    const SchedulingChangedCallback& callback) {
+  scheduling_changed_callback_ = callback;
 }
 
 Buffer GpuScheduler::GetSharedMemoryBuffer(int32 shm_id) {