Make File-Picker modal on Linux

Chromium for Linux uses GtkFileChooserDialog for opening a file-picker,
but it is not modal to the X11 host window because GtkFileChooserDialog
can be modal only to the parent GtkWindow.

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

Design doc and discussion:
https://docs.google.com/document/d/12CfKVTpaonxxM3sNksq6vY6qb0J2qR3b7h_bLxzYanE

This CL 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:
https://codereview.chromium.org/1594973009

BUG=408481,579408
TEST=BrowserSelectFileDialogTest.ModalTest

Review-Url: https://codereview.chromium.org/1624793002
Cr-Commit-Position: refs/heads/master@{#422482}
4 files changed