aura: Remove layerless windows.
Layerless windows are only used in unit tests today, and add
complexity in that a NativeWidgetAura can not rely on there
being a layer present. In practice, having a layer per Window
is not unreasonable overhead, and is simpler. The way to render
something into the same layer is to use a separate View instead
of a separate Window.
R=sky
BUG=466426
Review URL: https://codereview.chromium.org/1050713002
Cr-Commit-Position: refs/heads/master@{#323610}
53 files changed
