Make the C++ PlatformViewportAndroid's dtor call the Java PVA's detach() method.

In particular, after destruction of the C++ PVA, the Java PVA will no
longer try to call into it.

(Rename the previously-unused destroy() to detach(), and make it not
call into native code.)

BUG=Fixes #95
R=qsr@chromium.org

Review URL: https://codereview.chromium.org/1081443005
diff --git a/services/native_viewport/platform_viewport_android.h b/services/native_viewport/platform_viewport_android.h
index 75ed038..6592903 100644
--- a/services/native_viewport/platform_viewport_android.h
+++ b/services/native_viewport/platform_viewport_android.h
@@ -6,7 +6,6 @@
 #define SERVICES_NATIVE_VIEWPORT_PLATFORM_VIEWPORT_ANDROID_H_
 
 #include "base/android/jni_weak_ref.h"
-#include "base/android/scoped_java_ref.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "services/native_viewport/platform_viewport.h"
@@ -68,7 +67,8 @@
 
   void ReleaseWindow();
 
-  Delegate* delegate_;
+  Delegate* const delegate_;
+  JavaObjectWeakGlobalRef java_platform_viewport_android_;
   ANativeWindow* window_;
   mojo::ViewportMetricsPtr metrics_;
   ui::SequentialIDGenerator id_generator_;