IWYU for ax_virtual_view.cc

Commit d69d3bec added a call to ui::GetScaleFactorForNativeView() but not
an #include of ui/base/layout.h, which declares it.  This happens to
work right now via transitive #includes but broke an in-progress CL of
mine.

Bug: none
Change-Id: Id0d5f390052e6468b323958f2e2b713aef53ee16
Tbr: nektar
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2143519Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757788}

ui/views/accessibility/ax_virtual_view.cc

@@ -17,6 +17,7 @@
 #include "ui/accessibility/ax_action_data.h"
 #include "ui/accessibility/ax_tree_data.h"
 #include "ui/accessibility/platform/ax_platform_node.h"
+#include "ui/base/layout.h"
 #include "ui/base/ui_base_types.h"
 #include "ui/gfx/geometry/rect_conversions.h"
 #include "ui/views/accessibility/view_accessibility.h"