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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}