Fix a bug with throttling and timer queue suspension

components/scheduler/renderer/throttling_helper.cc

Index: components/scheduler/renderer/throttling_helper.cc
diff --git a/components/scheduler/renderer/throttling_helper.cc b/components/scheduler/renderer/throttling_helper.cc
index 31fcb7de54db1bfa34a454ad6f2705819ee7051a..7d0fdea9646e784c16a908e40cee6de34df98ae0 100644
--- a/components/scheduler/renderer/throttling_helper.cc
+++ b/components/scheduler/renderer/throttling_helper.cc
@@ -43,11 +43,26 @@ ThrottlingHelper::~ThrottlingHelper() {
   renderer_scheduler_->UnregisterTimeDomain(time_domain_.get());
 }
 
+void ThrottlingHelper::SetQueueEnabled(TaskQueue* task_queue, bool enabled) {
+  TaskQueueMap::iterator find_it = throttled_queues_.find(task_queue);
+
+  if (find_it == throttled_queues_.end()) {
+    task_queue->SetQueueEnabled(enabled);
+    return;
+  }
+
+  find_it->second.enabled = enabled;
+
+  if (!enabled)

[Sami, 2016/05/31 18:11:04]

Maybe add a comment why we don't want to enable the queue directly here? (it
could defeat throttling).

[alex clarke (OOO till 29th), 2016/06/02 13:51:05]

Done.

+    task_queue->SetQueueEnabled(false);
+}
+
 void ThrottlingHelper::IncreaseThrottleRefCount(TaskQueue* task_queue) {
   DCHECK_NE(task_queue, task_runner_.get());
 
   std::pair<TaskQueueMap::iterator, bool> insert_result =
-      throttled_queues_.insert(std::make_pair(task_queue, 1));
+      throttled_queues_.insert(std::make_pair(
+          task_queue, Metadata(1, task_queue->IsQueueEnabled())));
 
   if (insert_result.second) {
     // The insert was succesful so we need to throttle the queue.
@@ -64,20 +79,22 @@ void ThrottlingHelper::IncreaseThrottleRefCount(TaskQueue* task_queue) {
     }
   } else {
     // An entry already existed in the map so we need to increment the refcount.
-    insert_result.first->second++;
+    insert_result.first->second.throttling_ref_count++;
   }
 }
 
 void ThrottlingHelper::DecreaseThrottleRefCount(TaskQueue* task_queue) {
   TaskQueueMap::iterator iter = throttled_queues_.find(task_queue);
 
-  if (iter != throttled_queues_.end() && --iter->second == 0) {
+  if (iter != throttled_queues_.end() &&
+      --iter->second.throttling_ref_count == 0) {
+    bool enabled = iter->second.enabled;
     // The refcount has become zero, we need to unthrottle the queue.
     throttled_queues_.erase(iter);
 
     task_queue->SetTimeDomain(renderer_scheduler_->real_time_domain());
     task_queue->SetPumpPolicy(TaskQueue::PumpPolicy::AUTO);
-    task_queue->SetQueueEnabled(true);
+    task_queue->SetQueueEnabled(enabled);
   }
 }
 
@@ -120,7 +137,7 @@ void ThrottlingHelper::PumpThrottledTasks() {
     if (task_queue->IsEmpty())
       continue;
 
-    task_queue->SetQueueEnabled(true);
+    task_queue->SetQueueEnabled(map_entry.second.enabled);

[Sami, 2016/05/31 18:11:04]

Mind adding a DCHECK here that makes sure the enabled status is what we expect?

[alex clarke (OOO till 29th), 2016/06/02 13:51:05]

This in principle is a good idea, but due to the way
ThrottlingHelper::SetQueueEnabled operates it's a bit complex.  I think we can
get away without doing it.

     task_queue->PumpQueue(false);
   }
   // Make sure NextScheduledRunTime gives us an up-to date result.