Only support left/right maximizing at 50% width when the --ash-enable-alternate-caption-button

ash/accelerators/accelerator_controller.cc

Index: ash/accelerators/accelerator_controller.cc
diff --git a/ash/accelerators/accelerator_controller.cc b/ash/accelerators/accelerator_controller.cc
index d80cfea83c1a2deec1c5d574b61f31fa543acb62..02ed385737274d82a07318d07ea97629ef3c92e2 100644
--- a/ash/accelerators/accelerator_controller.cc
+++ b/ash/accelerators/accelerator_controller.cc
@@ -836,9 +836,12 @@ bool AcceleratorController::PerformAction(int action,
         break;
       }
 
-      internal::SnapSizer::SnapWindow(window,
-          action == WINDOW_SNAP_LEFT ? internal::SnapSizer::LEFT_EDGE :
-                                       internal::SnapSizer::RIGHT_EDGE);
+      using internal::SnapSizer;
+      if (SnapSizer::CanSnapWindow(window)) {
+        SnapSizer::Edge edge = (action == WINDOW_SNAP_LEFT) ?
+            SnapSizer::LEFT_EDGE : SnapSizer::RIGHT_EDGE;
+        SnapSizer::SnapWindow(window, edge, SnapSizer::STEP_YES);
+      }
       return true;
     }
     case WINDOW_MINIMIZE: {