Fix WindowModalityController's GetModalTransientChild[Ash]
This CL fixes an issue where menus would break dialog modality, allowing a
non-modal view to get focus, causing input to be blocked for the whole system.
This would arise when comboboxes were used in modal dialogs.
This has been fixed by making the modal transient child check
return the lowest modal transient in the window tree rather
than null when the leaf nodes of the tree weren't modal.
BUG=456697
Review URL: https://codereview.chromium.org/
1161233004
Cr-Commit-Position: refs/heads/master@{#333893}