fix(chunk_manager): fix serious memory leak.
fix(chunk_manager): fix serious memory leak.

Previously, chunks transferred into the frontend thread were never freed, due to the branches of an
if statement being reversed, with the consequence of memory growing without bound.

janelia-flyem/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,google/neuroglancer