Let's go w/ option #2. Quick mock attached. Close icon here:
https://icons.googleplex.com/#icon=ic_close

Let me know if you have any questions.

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Jonny, Hiro,

We need to define when to show the warning banner again.

For example, we can re-show the warning when:
1) XXX hours have passed but still low space.
2) A new Files app window is opened but still low space.
3) After the device space gets sufficient, the space becomes low again.
etc...

#2 is the easiest, but might be annoying?
#1 sounds better to me, but how many hours should we wait? (for example, 24 hours?)

To be clear, we need to define when to show the warning banner again AFTER the user dismisses the warning banner.

Can we tie this to the session? I think that's how we do the notification. Please ping dspaid@ for the exact logic. 

Chrome apps like Files app can't have per-session state by themselves.
Other services like cryptohome can send message on user's log-in, but adding communications with Files app and outer modules for this purpose sounds overkill to me.

Can we take other options which don't depend on user sessions for simplicity? (like options in c#2?)

+1 to following the same logic as the notification. If that's not possible then #1 and #3 SGTM (I agree #2 would get annoying). How does 36 hours sound?

Ok that's too bad. Let's go with #1 with a 36 hour loop. 

Thank you for accepting the alternative!
I'll go with #1 with 36 hour loop.

Currently we show the warning when the remaining space is less than 20%.
To be consistent with the notification, I think we should show it when the remaining space is less than 512MB or 1GB.
Which one do you think is better, 512MB or 1GB?

I think we should show it starting at 1GB.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/214729817d1ef92f27929c090a962f633b1e5716

commit 214729817d1ef92f27929c090a962f633b1e5716
Author: fukino <fukino@chromium.org>
Date: Thu Aug 25 10:00:14 2016

Files app: Make the warning banner about local space dismissable.

This CL includes following changes
- Add a dismiss button on warning banner about Downloads space. If a user dismiss the banner, we won't show unless 36 hours have passed.
- Show the warning banner about Downloads when the available size is less than 1GB. (It used to be 20%)

BUG= 634207 
TEST=manually tested by modifying DOWNLOADS_SPACE_WARNING_DISMISS_DURATION from 36 hours to 10 seconds.

Review-Url: https://codereview.chromium.org/2271203002
Cr-Commit-Position: refs/heads/master@{#414379}

[modify] https://crrev.com/214729817d1ef92f27929c090a962f633b1e5716/ui/file_manager/file_manager/foreground/css/file_manager.css
[modify] https://crrev.com/214729817d1ef92f27929c090a962f633b1e5716/ui/file_manager/file_manager/foreground/js/ui/banners.js

+Test team for verification. Please test this and  issue 640504  together when they are in 

Seen Low space banner on guest user account without any files on File.app.
Its reproduced on minnie but not on samus.

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For guest mode we mount tmpfs and we have only 1GB "free" on minnie.

+mitsuji@,
Do you think we should show the low-space notification in guest session?

Not much you can do while in guest mode so let's disable all notifications relating to disk full status. 

Relatedly we show low space notification for the guest user if the main user partition is low, even though guest is on tmpfs.

Notification referred to in Comment 18 is the system tray notification, not the files app banner.

Re:#17
I filed  issue 655879  to remove the notifications (from Files banner and system notification).

I'll get this issue back to Fixed, since the banner is still dismissable.

Verified on 8872.22.0, 55.0.2883.25

Low-space warning banner is dismissable.