Read compositor VSync information from platform, when possible

ash/display/display_controller.cc

Index: ash/display/display_controller.cc
diff --git a/ash/display/display_controller.cc b/ash/display/display_controller.cc
index 7b7cc5882e5592a336e718a28d2fa3c442bb5215..101e5af6b061b79ff0f6fc307f8fd1d24cdd0c0b 100644
--- a/ash/display/display_controller.cc
+++ b/ash/display/display_controller.cc
@@ -119,6 +119,15 @@ void SetDisplayPropertiesOnHostWindow(aura::RootWindow* root,
   scoped_ptr<aura::RootWindowTransformer> transformer(
       internal::CreateRootWindowTransformerForDisplay(root->window(), display));
   root->host()->SetRootWindowTransformer(transformer.Pass());
+
+  internal::DisplayMode mode;
+  if (GetDisplayManager()->GetSelectedModeForDisplayId(display.id(), &mode)) {

[brianderson, 2014/02/03 22:56:04]

Will this be called every frame? If not, how often will it be called?

[sheu, 2014/02/03 23:02:40]

This should be called only when monitor configuration changes.

[brianderson, 2014/02/04 01:39:51]

Does it happen in all the following cases:
When a window changes monitors.
When the monitor lowers its frequency to save power.
When the user manually selects a monitor config.

Also, does this code path only apply to ChromeOS?

[oshima, 2014/02/04 02:07:53]

No
Currently no, although it's not difficult to support this.
(just need to check refresh rate in DisplayManager::UpdateDisplays)
Yes
Yes

[brianderson, 2014/02/04 02:14:36]

Is that beca
For all instances where the answer is "No", is the answer "Yes" for the original
code path?

+    root->host()->compositor()->OnUpdateVSyncParameters(
+        base::TimeTicks(),

[brianderson, 2014/02/03 22:56:04]

Why isn't there timebase information here?

[sheu, 2014/02/03 23:02:40]

We don't have access to timebase information here, so we can't assign it a
value.

+        base::TimeDelta::FromMicroseconds(base::Time::kMicrosecondsPerSecond /
+                                          mode.refresh_rate),
+        true);

[brianderson, 2014/02/03 22:56:04]

Is this the only place where the authoritative interval is true?

[sheu, 2014/02/03 23:02:40]

Yes.

+  }
 }
 
 }  // namespace