[Chromoting] Use device::PowerSaveBlocker to block screen saver

remoting/host/basic_desktop_environment.cc

Index: remoting/host/basic_desktop_environment.cc
diff --git a/remoting/host/basic_desktop_environment.cc b/remoting/host/basic_desktop_environment.cc
index b047c26804112a1de221e3b3bae8d0bb15d495f6..cb62ff4677c4de3874a5a5f1f779c92774c4c01e 100644
--- a/remoting/host/basic_desktop_environment.cc
+++ b/remoting/host/basic_desktop_environment.cc
@@ -70,6 +70,8 @@ BasicDesktopEnvironment::CreateVideoCapturer() {
   DCHECK(caller_task_runner_->BelongsToCurrentThread());
 
   return base::WrapUnique(new DesktopCapturerProxy(video_capture_task_runner_,
+                                                   ui_task_runner_,
+                                                   caller_task_runner_,

[Hzj_jie, 2016/06/22 18:57:59]

We have not sent file_task_runner to BasicDesktopEnvironment, so I choose
network_task_runner (caller_task_runner_) instead. The IO blocking jobs are only
injected in PowerSaveBlocker's constructor and destructor, i.e.
DesktopCapturerProxy's constructor and destructor. Since there are no network
requests during that time, the impact would be small.

[Sergey Ulanov, 2016/06/22 19:05:37]

I don't think this is acceptable. We should never have any blocking tasks
running on network thread.

[Hzj_jie, 2016/06/24 19:14:29]

I think all the jobs in network thread are all IO jobs, thus type of
network_task_runner is TYPE_IO. But I also found a DisallowBlockingOpertions has
been applied to it, which looks weird. Can I assume only asynchronized jobs are
executing on a network_task_runner?

[Sergey Ulanov, 2016/06/27 18:20:24]

That's right, we only allow non-blocking IO on that thread.

                                                    *desktop_capture_options_));
 }