Mac: Remove extension dropdown from folder dialogs.
Mac: Remove extension dropdown from folder dialogs.

Currently, file extension dropdowns are displayed on Mac for
SELECT_FOLDER and SELECT_UPLOAD_FOLDER dialogs if the SelectFile API
client provides the extension list. This CL fixes this by not showing an
extension dropdown for folder dialogs.

BUG=564448

Review URL: https://codereview.chromium.org/1531223002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#366690}

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