[MacViews] Fix NativeWidgetMacTest.MiniaturizeExternally.
[MacViews] Fix NativeWidgetMacTest.MiniaturizeExternally.

This broke after r323400 and r323610 because layer-less widgets were no
longer supported.

This changes the test to assume a layered widget. Since painting is now
asynchronous, pump the message loop after each UI action.

BUG=378134

Review URL: https://codereview.chromium.org/1266313002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#341493}

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