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The behaviour is similar to the file pickers on other platforms.

When clicking "OK", a file extension is only added if:
 - A FileType filter is active that has a default extension (i.e.
     not the all files filter, or an extensionless filter), and
 - The user-provided filename does not already have an extension.

The file picker already has logic for adding extensions, but it's only
applied when changing the currently active filter in the UI (and only
when the filename already has an extension that isn't *any* of the
possible extensions for the current filter). Repurpose it.

The approach taken updates the UI field when clicking close, so the user
briefly sees the extension being added before the dialog dismisses.

To keep logic & tests simple/robust it was necessary to ensure filters
are applied in the dialog correctly when there is only one filter
available, which regressed recently (https://crbug.com/1097448).

Bug: 1082624, 1097448, 1097633
Change-Id: I1488880be8c289853b864d4bea0e193beb5464ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2255918
Commit-Queue: Trent Apted <tapted@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781157}