Make File-Picker modal on Linux

Chromium for Linux opens a Gtk file-picker for file browsing, but it is not
modal because there is no GtkWindow as parent of file-picker.

This patch allows the X11 host window to disable input event handling to make
a file-picker modal

The original CL was reverted due to wrong DCHECK with modal_dialog_xid_:

TEST=BrowserSelectFileDialogTest, ModalTest

Review URL:

Cr-Commit-Position: refs/heads/master@{#360961}
5 files changed