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WindowsEventRouter needs to broadcast when no Chrome window has focus.

When toolkit_views first flips on Mac, nothing should actually change in
the release.

Until Chrome on Mac *only* makes Windows with toolkit-views,
WindowsEventRouter on Mac should still use
NSWindowDidBecomeKeyNotification + NOTIFICATION_NO_KEY_WINDOW.

This works both for windows created with toolkit-views and those created
natively.

BUG=439882

Review URL: https://codereview.chromium.org/773303005

Cr-Commit-Position: refs/heads/master@{#308452}