This is needed for PHC (bug 1523276).

jemalloc_replace_dynamic() is badly broken. If you install a malloc table
other than the default at startup (e.g. DMD's or PHC's), when you call
jemalloc_replace_dynamic() it installs a new allocator that wraps the
default allocator, and then when you call jemalloc_replace_dynamic(nullptr)
it switches back to the default allocator.

This commits makes numerous improvements.

• It removes the "flip-flopping" between malloc tables, which didn't really
  work and isn't necessary.

• jemalloc_replace_dynamic() now switches between the original malloc table
  and the new one, rather than the default malloc table and the new one.

• It renames various things, to make the names shorter and clearer.

• It clearly documents the dangers and limitations of
  jemalloc_replace_dynamic().

• It removes and inlines profiler::Init(), because there was only one call
  site.

• It rearranges install_memory_counter() so the control flow is simpler.

https://hg.mozilla.org/mozilla-central/rev/e07990ce888f