(Merge to M-51) Revise the shelf alignment locking mechanism.

chrome/browser/ui/ash/launcher/chrome_launcher_controller.cc

Index: chrome/browser/ui/ash/launcher/chrome_launcher_controller.cc
diff --git a/chrome/browser/ui/ash/launcher/chrome_launcher_controller.cc b/chrome/browser/ui/ash/launcher/chrome_launcher_controller.cc
index f7b747d0e6debdc473c13c07f86e89a486d0bbff..bdc1482c7a5f1d83d3e23141c3e232ea88433cc0 100644
--- a/chrome/browser/ui/ash/launcher/chrome_launcher_controller.cc
+++ b/chrome/browser/ui/ash/launcher/chrome_launcher_controller.cc
@@ -1164,6 +1164,10 @@ void ChromeLauncherController::OnShelfAlignmentChanged(ash::Shelf* shelf) {
     case ash::SHELF_ALIGNMENT_RIGHT:
       value = ash::kShelfAlignmentRight;
       break;
+    case ash::SHELF_ALIGNMENT_BOTTOM_LOCKED:
+      // This should not be a valid preference option for now. We only want to
+      // lock the shelf during login or when adding a user.
+      return;
   }
 
   aura::Window* root_window =