Remove unused <memory> header

Bug: None
Test: None
Change-Id: I729a255a07d9455530819edbc9ed6508cf0bfee1
Reviewed-on: https://chromium-review.googlesource.com/630797
Reviewed-by: David Reveman <reveman@chromium.org>
Commit-Queue: Daichi Hirono <hirono@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496910}

components/exo/wm_helper.h

@@ -5,8 +5,6 @@
 #ifndef COMPONENTS_EXO_WM_HELPER_H_
 #define COMPONENTS_EXO_WM_HELPER_H_
 
-#include <memory>
-
 #include "base/macros.h"
 #include "base/observer_list.h"
 #include "ui/aura/client/drag_drop_delegate.h"