Reorder AudioMessageFilter shutdown. Remove spammy DCHECK.
Fixes NULL pointer dereference when the RenderThreadImpl is
destructed but outstanding HTMLMediaElement objects have not
yet been garbage collected.
Reordering will keep audio streams alive slightly longer, but
that should be of no consequence since they would still respond
to graceful shutdown attempts by any living AudioRendererImpls.
BUG=408345
TEST=none
Review URL: https://codereview.chromium.org/
517793003
Cr-Commit-Position: refs/heads/master@{#293999}