Adding a way to associate Android Activities with PlatformWindowAndroid

ui/aura/window_tree_host_platform.cc

Index: ui/aura/window_tree_host_platform.cc
diff --git a/ui/aura/window_tree_host_platform.cc b/ui/aura/window_tree_host_platform.cc
index 2a7d0541a744fa9e5747798d2f57699cc918aac6..59398808386fe2c05c6a52f4ca391545571978a4 100644
--- a/ui/aura/window_tree_host_platform.cc
+++ b/ui/aura/window_tree_host_platform.cc
@@ -11,6 +11,8 @@
 #include "ui/gfx/screen.h"
 
 #if defined(OS_ANDROID)
+#include "base/android/jni_android.h"
+#include "ui/aura/client/aura_constants.h"
 #include "ui/platform_window/android/platform_window_android.h"
 #endif
 
@@ -72,6 +74,18 @@ gfx::AcceleratedWidget WindowTreeHostPlatform::GetAcceleratedWidget() {
 }
 
 void WindowTreeHostPlatform::ShowImpl() {
+#if defined(OS_ANDROID)
+  JNIEnv* env = base::android::AttachCurrentThread();
+  ui::PlatformWindowAndroid* pwa =
+      ui::PlatformWindowAndroid::From(window_.get());
+  if (!pwa->IsAttachedToActivity()) {
+    base::android::ScopedJavaLocalRef<jobject> activity =
+        window()->GetProperty(aura::client::kActivityForRootWindow)->get(env);
+    if (activity.is_null())
+      return;
+    pwa->AttachToActivity(env, activity);
+  }
+#endif
   window_->Show();
 }