Mac: do not enable file upload until selection has changed

To avoid files being uploaded without the user realizing, require the
user to change the selection on the file dialog before the OK button
is enabled.

Bug: 637098
Change-Id: Ic6faffb3f6075fb969ae930992d2553162346d02
Reviewed-on: https://chromium-review.googlesource.com/833399
Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: Emily Stark <estark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525148}
1 file changed