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This is a reland of 2ed82a41

Original change's description:
> Set Xlib error handler after GTK X11 initialization
>
> The default GTK error handler exits the process, which we don't want.
> We used to reset the error handler at some point after GTK
> initialization, but this became lost after r815319.
>
> Also refactor the XlibLoader instance to be global so it doesn't have
> to be created multiple times for each connection.
>
> R=sky
>
> Bug: 1145929
> Change-Id: I73a3f27551534c2262bd8994d33e30fc2a0a4e27
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2527743
> Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
> Reviewed-by: Scott Violet <sky@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#825585}

Bug: 1145929
Change-Id: Ib65f3add91181b3bf82dd71f7b9b87a9a681a53d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533199
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826572}