Allow native_viewport to create new native windows on demand on Android.

services/native_viewport/platform_viewport.h

Index: services/native_viewport/platform_viewport.h
diff --git a/services/native_viewport/platform_viewport.h b/services/native_viewport/platform_viewport.h
index 69bc4c7b2624f2017df531061a9b8e368eebdb77..e518dfafa2aa7b6d8f4fa4ed97ae493a3e0e01e0 100644
--- a/services/native_viewport/platform_viewport.h
+++ b/services/native_viewport/platform_viewport.h
@@ -41,6 +41,11 @@ class PlatformViewport {
   virtual gfx::Size GetSize() = 0;
   virtual void SetBounds(const gfx::Rect& bounds) = 0;
 
+  // Called when a new native surface has been created by the host system and
+  // should be used by this viewport. Returns whether the surface has been
+  // attached to or not.
+  virtual bool NewNativeSurfaceAvailable() = 0;
+
   static scoped_ptr<PlatformViewport> Create(Delegate* delegate);
 };