MacViews: Allow views::Widgets to be parented off NSWindows

Currently, a views::Widget on Mac can't be owned by a NSWindow that is
not also a views::Widget. This would mean that to add a toolkit-views
dialog parented to the browser, the whole browser window would need to
be ported to views first.

This CL abstracts the responsibilities of a NativeViewMac's parent
window into a BridgedNativeWidgetOwner interface which
BridgedNativeWidget implements to retain the current behaviour.

WidgetOwnerNSWindowAdapter is a lightweight class that implements this
to allow the parent to be any native NSView. The parent window closing
will also close the child, and (for appropriate child types) the child
window frame is offset by the parent view's position on screen.

BUG=447086
TEST=views_unittests' NativeWidgetMacTest.NonWidgetParent

Review URL: https://codereview.chromium.org/1063933003

Cr-Commit-Position: refs/heads/master@{#329097}

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