device_orientation::DispatcherHost: one Provider observer per render_view_id.

chrome/browser/device_orientation/provider.h

Index: chrome/browser/device_orientation/provider.h
diff --git a/chrome/browser/device_orientation/provider.h b/chrome/browser/device_orientation/provider.h
index f32b0f6b527004210c2713c2bca3f6dd233d2a60..c6b1b7aba68fc2e9a32379aee764111bf1f92588 100644
--- a/chrome/browser/device_orientation/provider.h
+++ b/chrome/browser/device_orientation/provider.h
@@ -33,6 +33,8 @@ class Provider : public base::RefCountedThreadSafe<Provider> {
   // injected object's reference count.
   static void SetInstanceForTests(Provider* provider);
 
+  // Note: AddObserver may call back synchronously to the observer with
+  // orientation data.
   virtual void AddObserver(Observer* observer) = 0;
   virtual void RemoveObserver(Observer* observer) = 0;