Remove insignificant message: Default shared memory cache: Cache named pagespeed_default_shm already exists

[GoogleCodeExporter]

Allow multiple calls to CreateShmMetadataCache(default) without giving an error

Users of 1.7.30.1 are seeing 'Default shared memory cache: Cache named
pagespeed_default_shm already exists' in their error log.  This is not
actually a problem, and is caused by LookupShmMetadataCache returning
NULL for a given name even after a successful call with that same name
to CreateShmMetadataCache, due to LookupShmMetadataCache needing
cache_to_use to be set (which doesn't happen until RootInit).  Just
silently accept these additional calls to CreateShmMetadataCache and
do nothing.

Original issue reported on code.google.com by hui...@google.com on 6 Jan 2014 at 4:33