Un-register the PassthroughGLDebugMessageCallback on Destroy

When the passthrough command decoder is destroyed, the callback needs
to be un-registered from ANGLE. That way if the ANGLE context is used
elsewhere, it won't dereferences an invalid pointer.

Bug: chromium:1048205
Change-Id: Icf1b9969fa2bbc53d449e1b73ef66853303e94c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2055423Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Cr-Commit-Position: refs/heads/master@{#745063}

gpu/command_buffer/service/gles2_cmd_decoder_passthrough.cc

@@ -1323,6 +1323,10 @@ void GLES2DecoderPassthroughImpl::Destroy(bool have_context) {
     group_ = nullptr;
   }
 
+  if (have_context) {
+    api()->glDebugMessageCallbackFn(nullptr, nullptr);
+  }
+
   if (context_.get()) {
     context_->ReleaseCurrent(nullptr);
     context_ = nullptr;