Revert of Make File-Picker modal on Linux (patchset #18 id:340001 of )

Reason for revert:
Breaks input events in some cases (

Original issue's description:
> 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_:
> BUG=408481
> TEST=BrowserSelectFileDialogTest, ModalTest
> Committed:
> Cr-Commit-Position: refs/heads/master@{#360961},,,,
BUG=408481, 579408

Review URL:

Cr-Commit-Position: refs/heads/master@{#370570}
(cherry picked from commit 0bf45553ab62cc5c97ee24bcdfbba9a0526235ea)

Review URL: .

Cr-Commit-Position: refs/branch-heads/2623@{#36}
Cr-Branched-From: 92d77538a86529ca35f9220bd3cd512cbea1f086-refs/heads/master@{#369907}
5 files changed