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After [1], gfx::GetXDisplay() returns a different display for each
thread.  This introduces a race condition since |g_handler| is shared
across all threads.

Since X11ErrorHandler will be removed soon anyway (since XProto uses a
different error-handling mechanism), this CL simply removes the
DCHECK().

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2343314

BUG=1122006
R=sky

Change-Id: I1101a3456f4609481a1f204634102dac89bad0ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2402304
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@{#805585}