Make the library usable for brightray and chat on Linux

libchromiumcontent builds with a tcmalloc dependency by default.  It comes with its own tcmalloc implementation but doesn't seem to compile it.  So I've just disabled the tcmalloc dependency.  This will also save us from oddities where a binary or library linking to libchromiumcontent uses a different, incompatible malloc.

Binaries based on libchromiumcontent need the SUID sandbox, `chrome_sandbox`.  So now it gets built and put in the artifact .zip.