Moves TransientWindowClient to ui/aura/client and adds observer

Mus is going to need to use this from within aura, so I'm moving it to
ui/aura/client. Additionally I'm moving creation of WmState to
AuraTestHelper. Again, mus is going to need it aura test code already
depends upon ui/wm, so I can't think of a reason not to do it
everywhere.

BUG=659155
TEST=covered by tests
R=ben@chromium.org

Review-Url: https://codereview.chromium.org/2453953003
Cr-Commit-Position: refs/heads/master@{#428087}

• ash/test/ash_test_helper.cc[diff]
• ash/test/ash_test_helper.h[diff]
• content/browser/renderer_host/render_widget_host_view_aura.cc[diff]
• content/shell/browser/shell.h[diff]
• content/shell/browser/shell_views.cc[diff]
• ui/aura/BUILD.gn[diff]
• ui/aura/client/transient_window_client.cc[Added - diff]
• ui/aura/client/transient_window_client.h[Added - diff]
• ui/aura/client/transient_window_client_observer.h[Added - diff]
• ui/aura/test/DEPS[diff]
• ui/aura/test/aura_test_helper.cc[diff]
• ui/aura/test/aura_test_helper.h[diff]
• ui/views/corewm/tooltip_controller_unittest.cc[diff]
• ui/views/test/test_views_delegate.h[diff]
• ui/views/test/test_views_delegate_aura.cc[diff]
• ui/views/test/views_test_helper_aura.cc[diff]
• ui/views/test/views_test_helper_aura.h[diff]
• ui/wm/core/easy_resize_window_targeter.cc[diff]
• ui/wm/core/focus_controller_unittest.cc[diff]
• ui/wm/core/shadow_controller_unittest.cc[diff]
• ui/wm/core/transient_window_controller.cc[diff]
• ui/wm/core/transient_window_controller.h[diff]
• ui/wm/core/transient_window_manager.cc[diff]
• ui/wm/core/transient_window_manager_unittest.cc[diff]
• ui/wm/core/wm_state.h[diff]
• ui/wm/test/wm_test_helper.cc[diff]
• ui/wm/test/wm_test_helper.h[diff]