Fix userVisibility for files in ChromeOS filesystems.

In Web applications, the Filesystem API is used to create sandboxed
filesystems. Files in these filesystems should never be shown in a
filepicker. ChromeOS supports a few special filesystems (Downloads,
Gallery) where the files should be shown in filepickers.

This change fixes the userVisibility property for File instances created
from such filesystems.

BUG=398366

Review URL: https://codereview.chromium.org/516763002

git-svn-id: svn://svn.chromium.org/blink/trunk@181530 bbb929c8-8fbe-4397-9dbb-9b2b20218538