Enable to open multiple file-pickers in Chromium for Linux

Sometimes, we don't know exact requirements or expect user behaviors while developing or fix something. I fixed the file-picker modal issue in Chromium for Linux last year. At that time, the reviewer and I thought that there would be no case to pop-up multiple file-pickers, but it happened  since M55 when you follow the following steps:
1) Enable "Ask where to save" in settings.
2) Open 2 tabs of e.g.: https://sourceforge.net/projects/azureus/files/latest/download
3) Wait for 2 downloader windows, and close/cancel/etc both
4) Freeze or crash

Here is a video to reproduce the problem.



When file-picker is opened, it disables event listening of the main host window. Then, the user closes the file-picker, it enables the event listening. Now, the host widow has a counter to check the number of the open file-pickers and it doesn't disable the event listening if there are any open file-picker. The event listening can be enabled when the last file-picker is closed. Here is the fix.
Anyway, you may see the fix in M58(Apr 25th, 2017)