Presentation API: allow the Mojo service to reject a getAvailability() call.

content/common/presentation/presentation_service.mojom

Index: content/common/presentation/presentation_service.mojom
diff --git a/content/common/presentation/presentation_service.mojom b/content/common/presentation/presentation_service.mojom
index d033a482cb80d280926902f9261a8e850ebbc8f2..1bdf39b498e1c0bc587e46d60d7561f5a534f65a 100644
--- a/content/common/presentation/presentation_service.mojom
+++ b/content/common/presentation/presentation_service.mojom
@@ -95,7 +95,7 @@ interface PresentationService {
   // the send requests and invalidate all pending requests. This occurs
   // for eg., when frame is deleted or navigated away.
   SendSessionMessage(SessionMessage message_request) => (bool success);
-  
+
   // Called when closeSession() is called by the frame.
   CloseSession(string presentation_url, string presentation_id);
 
@@ -114,5 +114,15 @@ interface PresentationService {
 };
 
 interface PresentationServiceClient {
+  // Called when the client tries to listen for screen availability changes but

[whywhat, 2015/07/06 18:27:18]

nit: Rephrase as

"Called when the client starts listening for screen availability and passive
availability monitoring is not supported by the device either permanently
(hardware limitations) or temporarily (battery saver mode)."

Should we indicate if the limitation is temporarily or not so the page/Blink
layer could try again depending on battery status, for example?

+  // it is not supported by the device or underlying platform. This can also be
+  // called if the device is currently in a mode where it can't do screen
+  // discoveries (eg. low battery).
+  OnScreenAvailabilityNotSupported();
+
+  // Called when the client is listening for screen availability and the state

[whywhat, 2015/07/06 18:27:18]

nit: "Called immediately when the client starts listening for screen
availability if the current availability state is known and later to notify the
client of the state updates."

+  // changes. When the client starts to listen for screen availability, this
+  // method will always be called to give the current known state. It will then
+  // be called to notify of state updates.
   OnScreenAvailabilityUpdated(bool available);
 };