[XProto] Remove bad DCHECK in x11_error_tracker.cc

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}

ui/gfx/x/x11_error_tracker.cc

@@ -11,7 +11,6 @@
 namespace {
 
 unsigned char g_x11_error_code = 0;
-static gfx::X11ErrorTracker* g_handler = nullptr;
 
 int X11ErrorHandler(Display* display, XErrorEvent* error) {
   g_x11_error_code = error->error_code;
@@ -23,17 +22,12 @@ int X11ErrorHandler(Display* display, XErrorEvent* error) {
 namespace gfx {
 
 X11ErrorTracker::X11ErrorTracker() {
-  // This is a non-exhaustive check for incorrect usage. It disallows nested
-  // X11ErrorTracker instances on the same thread.
-  DCHECK(g_handler == nullptr);
-  g_handler = this;
   XSync(GetXDisplay(), x11::False);
   old_handler_ = reinterpret_cast<void*>(XSetErrorHandler(X11ErrorHandler));
   g_x11_error_code = 0;
 }
 
 X11ErrorTracker::~X11ErrorTracker() {
-  g_handler = nullptr;
   XSetErrorHandler(reinterpret_cast<XErrorHandler>(old_handler_));
 }