views/mus: Fix input event handling.
views/mus: Fix input event handling.

Input event handling moved from PlatformWindowMus to NativeWidgetMus
in https://crrev.com/394564, but NativeWidgetMus wasn't installing
itself as the input-event handler for the mus::Window.

BUG=609555

Review-Url: https://codereview.chromium.org/1990333002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#394860}

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