Wire up Android back button in SkyShell

sky/shell/android/org/domokit/sky/shell/PlatformViewAndroid.java

Index: sky/shell/android/org/domokit/sky/shell/PlatformViewAndroid.java
diff --git a/sky/shell/android/org/domokit/sky/shell/PlatformViewAndroid.java b/sky/shell/android/org/domokit/sky/shell/PlatformViewAndroid.java
index 8c228a0d737ad96a39a899b8ec641fa039772b0b..1a036fb6be818d60cb15cff733aa2a435d547779 100644
--- a/sky/shell/android/org/domokit/sky/shell/PlatformViewAndroid.java
+++ b/sky/shell/android/org/domokit/sky/shell/PlatformViewAndroid.java
@@ -176,6 +176,12 @@ public class PlatformViewAndroid extends SurfaceView
         mViewportObserver.onInputEvent(event);
     }
 
+    public void onBackPressed() {
+        InputEvent event = new InputEvent();
+        event.type = EventType.BACK;
+        mViewportObserver.onInputEvent(event);
+    }
+
     public void loadUrl(String url) {
         mViewportObserver.loadUrl(url);
     }