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:

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:


Cr-Commit-Position: refs/heads/master@{#422482}
4 files changed