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Since Chrome 8, invoking <input type="file"> in a browser action popup
on OSX would show, then immediately close the sheet (and the popup).

This CL fixes it by ignoring loss of key status on the bubble window
when it has an attached sheet. Loss of key status (or clicks) on windows
that are sheets themselves are also ignored, otherwise dismissing the
sheet would close the bubble as well, since it's already lost its "sheet
attached" status by that stage.

BUG=61632

Review URL: https://codereview.chromium.org/576643002

Cr-Commit-Position: refs/heads/master@{#295198}