Use UserWorkAreaBounds for calculating tray size

This should fix the size of trays when the docked content is above the
desktop (eg. docked magnifier, spoken feedback, etc...)

Bug: 824077
Test: manual
Change-Id: Iecccb721b14a2ffde045a87993d4951cd677efa4
Reviewed-on: https://chromium-review.googlesource.com/981846
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546078}

ash/system/web_notification/web_notification_tray.cc

@@ -354,9 +354,9 @@ bool WebNotificationTray::ShowMessageCenterInternal(bool show_settings,
 
     // In the horizontal case, message center starts from the top of the shelf.
     // In the vertical case, it starts from the bottom of WebNotificationTray.
-    const int max_height =
-        (shelf()->IsHorizontalAlignment() ? shelf()->GetIdealBounds().y()
-                                          : GetBoundsInScreen().bottom());
+    const int max_height = (shelf()->IsHorizontalAlignment()
+                                ? shelf()->GetUserWorkAreaBounds().height()
+                                : GetBoundsInScreen().bottom());
     // Sets the maximum height, considering the padding from the top edge of
     // screen. This padding should be applied in all types of shelf alignment.
     message_center_bubble->SetMaxHeight(max_height - kPaddingFromScreenTop);