Fix calculation of the bubble view height.

BUG=678558
TEST=Notification doesn't overlap with Uber Tray when ChromeVox is
     enabled.

Review-Url: https://codereview.chromium.org/2625663002
Cr-Commit-Position: refs/heads/master@{#442756}

ash/common/system/tray/system_tray.cc

@@ -682,7 +682,7 @@ void SystemTray::UpdateWebNotifications() {
             ->GetDisplayNearestWindow(bubble_view->GetWidget()->GetNativeView())
             .work_area();
     height =
-        std::max(0, work_area.height() - bubble_view->GetBoundsInScreen().y());
+        std::max(0, work_area.bottom() - bubble_view->GetBoundsInScreen().y());
   }
   if (web_notification_tray_)
     web_notification_tray_->SetTrayBubbleHeight(height);