Implement PermissionService::GetNextPermissionChange.

content/common/permission_service.mojom

Index: content/common/permission_service.mojom
diff --git a/content/common/permission_service.mojom b/content/common/permission_service.mojom
index 2ed9d3eb837e9316ec2ac281af817d6d3cd7d15a..16d4866355cc354779ad887a2b4eefc2b0136498 100644
--- a/content/common/permission_service.mojom
+++ b/content/common/permission_service.mojom
@@ -24,4 +24,13 @@ interface PermissionService {
     => (PermissionStatus status);
   RevokePermission(PermissionName permission, string origin)
     => (PermissionStatus status);
+
+  // Runs the callback next time there is a permission status change for the
+  // given { permission, origin }. Callers of this method will have to call it
+  // again if they want to keep listening to the changes. To prevent race
+  // conditions, the caller must pass the last known value.

[Tom Sepez, 2015/03/11 17:18:42]

Doesn't this imply that the service provider might have to buffer a potentially
infinite set of changes since a request for the next one might arrive at any
point in the future?

[mlamouri (slow - plz ping), 2015/03/18 16:24:40]

No. It means that the service provider will do a check when it receives the call
and if the value it gets is different from the one being passed, it will
directly run the callback with the new value instead of observing. It will only
observe for changes if the current value is different from the last known value
by the caller.

+  GetNextPermissionChange(PermissionName permission,
+                          string origin,
+                          PermissionStatus last_known_status)
+    => (PermissionStatus status);
 };