Aura: Fix linux component build

ui/aura_shell/activation_controller.h

Index: ui/aura_shell/activation_controller.h
diff --git a/ui/aura_shell/activation_controller.h b/ui/aura_shell/activation_controller.h
index 226c26c0ce86cd653db5ed570211db2971a9acb8..0a4e1290dbc2def96e532a8a4cbad19e73de02a3 100644
--- a/ui/aura_shell/activation_controller.h
+++ b/ui/aura_shell/activation_controller.h
@@ -12,13 +12,15 @@
 #include "ui/aura/client/activation_client.h"
 #include "ui/aura/root_window_observer.h"
 #include "ui/aura/window_observer.h"
+#include "ui/aura_shell/aura_shell_export.h"
 
 namespace aura_shell {
 namespace internal {
 
-class ActivationController : public aura::ActivationClient,
-                             public aura::WindowObserver,
-                             public aura::RootWindowObserver {
+// Exported for unit tests.
+class AURA_SHELL_EXPORT ActivationController : public aura::ActivationClient,
+                                               public aura::WindowObserver,
+                                               public aura::RootWindowObserver {
  public:
   ActivationController();
   virtual ~ActivationController();