I'll do this.  Doing a lot of small fixes this week, this seems to fit the bill.

Reassigning to eroman, who suggested this as a starter but for Yixin.

Note that the fix should handle cancellation, multiple messages sent from JS at once (Like a double click on the button before we manage show the modal dialog).  Not sure if it's possible to reasonably write tests for this or not.

So, to go into a bit more detail:

The problem is that the select_file_dialog_ refptr is never cleared.  So to fix this, we need to clear it when the dialog is closed (Both when it's cancelled without selecting a path, and when a path is selected.  Not that there's currently no callback implemented by net-export in the cancellation case).

Also, where the DCHECK is now, we should instead not show the dialog again if select_file_dialog_ is non-NULL.  With the above change, that will only happen if the user manages to click the button twice before we show the dialog once.  We really don't want to show two dialogs at once, so should just not show a dialog when that happens.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bfe6949954d4ed616ae69479b17f79142317c722

commit bfe6949954d4ed616ae69479b17f79142317c722
Author: wangyix <wangyix@google.com>
Date: Tue Oct 25 18:38:25 2016

- Removed a DCHECK that prevented "start logging" a second time.
- Parent window is now blocked when the save dialog pops up.
- The SelectFileDialog instance is now cleaned up as soon as the user saves or cancels.

BUG= 657219 

Review-Url: https://codereview.chromium.org/2440173003
Cr-Commit-Position: refs/heads/master@{#427422}

[modify] https://crrev.com/bfe6949954d4ed616ae69479b17f79142317c722/chrome/browser/ui/webui/net_export_ui.cc