cros: Don't process events on app icons while the shelf is hidden.

2 bug fixes.
1. Fix swiping up on the auto-hidden shelf from interacting with
   ShelfButtons.
2. Prevent users from accidentally activating ShelfButtons by tapping
   the bottom of the screen while the shelf is autohidden.

Bug: 872854
Change-Id: I5376eac2b7090b6109ad28cb0e9dfc4a748343e6
Reviewed-on: https://chromium-review.googlesource.com/1198522
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588101}

ash/shelf/shelf_layout_manager.cc

@@ -404,6 +404,13 @@ bool ShelfLayoutManager::ProcessGestureEvent(
 
   if (gesture_drag_status_ != GESTURE_DRAG_IN_PROGRESS &&
       gesture_drag_status_ != GESTURE_DRAG_APPLIST_IN_PROGRESS) {
+    if ((shelf_->GetVisibilityState() == SHELF_AUTO_HIDE &&
+         shelf_->GetAutoHideState() == SHELF_AUTO_HIDE_HIDDEN) ||
+        shelf_->GetVisibilityState() == SHELF_HIDDEN) {
+      // Do not allow children to handle events while the shelf is hidden.
+      return true;
+    }
+
     return false;
   }