Make file-picker modal in Chromium for Linux

When I stared working on Chromium, I found a bug in Chromium for Linux: the file-picker was not modal. So, while opening a file-picker, the users could control the main window. Sometimes, it caused mis-behaviors such as we were able to send an email while attaching a file.
There was already a bug and it seemed easy to fix it, but, it took 2 years to fully fix the problem.

The root cause is as follows:

Chromium for Linux uses GtkFileChooserDialog to open a file-picker, but it is not modal to the X11 host window because GtkFileChooserDialog can be modal only to the parent GtkWindow. So I tried to allow the X11 host window to disable input event handling to make a file-picker modal:  Here is the details:

Opening a file-picker

DisableEventListening() disables event listening for the host window using aura::ScopedWindowTargeter, which allows to temporarily replace the event-targeter with ui::NullEventTargeter. It returns a scope handle that is used to call |destroy_callback| when closing the file-picker.

class ScopedHandle {
 public:
   explicit ScopedHandle(const base::Closure& destroy_callback);
   ~ScopedHandle();
   void CancelCallback();

 private:
    base::Closure destroy_callback_;
    DISALLOW_COPY_AND_ASSIGN(ScopedHandle);
};

In addition, we also set another destroy callback(OnFilePickerDestroy) to the GtkFileChooserDialog that can be called when the file-picker is closed.

Close the file-picker

As you can see, OnFilePickerDestroy deletes scoped_handle.

void OnFilePickerDestroy(views::DesktopWindowTreeHostX11::ScopedHandle*

   scoped_handle) {

 delete scoped_handle;

}

Then, |destroy_callback| of ScopedHandle below is automatically called.

void DesktopWindowTreeHostX11::EnableEventListening() {

 DCHECK(modal_dialog_xid_);

 modal_dialog_xid_ = 0;

 targeter_for_modal_.reset();

}

You can find more details and discussion at here.

The first change list was reverted due to the UI freezing problem that happens when the users open a file-picker from a child window of the X11 host window. The second change list finally fixed this issue(BUG 408481, 579408). I also added a test case for the fix: BrowserSelectFileDialogTest.ModalTest.