chromeos: Rename ash::WmShelf to Shelf

ash/system/overview/overview_button_tray.cc

Index: ash/system/overview/overview_button_tray.cc
diff --git a/ash/system/overview/overview_button_tray.cc b/ash/system/overview/overview_button_tray.cc
index f4fb1ded48ea4c54c331e5c8fca5c43e9b85dc8f..fe543b53f23ac977d2fadb11e2b94710217e3a84 100644
--- a/ash/system/overview/overview_button_tray.cc
+++ b/ash/system/overview/overview_button_tray.cc
@@ -23,8 +23,8 @@
 
 namespace ash {
 
-OverviewButtonTray::OverviewButtonTray(WmShelf* wm_shelf)
-    : TrayBackgroundView(wm_shelf),
+OverviewButtonTray::OverviewButtonTray(Shelf* shelf)
+    : TrayBackgroundView(shelf),
       icon_(new views::ImageView()),
       scoped_session_observer_(this) {
   SetInkDropMode(InkDropMode::ON);