Don't crash in AsyncLTFS::OnMojoConnectionError

SingleClientSessionsSyncTest.NavigationChainAlteredDestructively has a
lot of flakes from this fatal log. Until the root cause for this
particular crash is found, use DLOG(ERROR) to avoid a crash.

This should only affect debug and canary builds.

Bug: 992207, 1043899
Change-Id: I53c1fbfbbfe76e4271ba873434e9cd4e3fee92c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2374512Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Sean Gilhuly <sgilhuly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801367}

cc/mojo_embedder/async_layer_tree_frame_sink.cc

@@ -356,8 +356,9 @@ void AsyncLayerTreeFrameSink::OnNeedsBeginFrames(bool needs_begin_frames) {
 void AsyncLayerTreeFrameSink::OnMojoConnectionError(
     uint32_t custom_reason,
     const std::string& description) {
+  // TODO(sgilhuly): Use DLOG(FATAL) once crbug.com/1043899 is resolved.
   if (custom_reason)
-    DLOG(FATAL) << description;
+    DLOG(ERROR) << description;
   if (client_)
     client_->DidLoseLayerTreeFrameSink();
 }