Fixes crashes and other bad behavior with modal dialogs. Maintains that transient children of docked windows or attached panels are parented in the same container as their transient parents. Ignores transient children for the purposes of panel or docked layout.
Fixes the stacking order of groups with transient children to move all descendants of common siblings when stacking above or below other windows.

BUG=297974
BUG=297977
TEST=follow the steps in bug repro.
TEST=ash_unittests --gtest_filter=*DragWindowWithTransientChild*
TEST=ash_unittests --gtest_filter=*PanelWithTransientChild*

Review URL: https://codereview.chromium.org/24450003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226881 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226881