Move Source/core/loader/archive to Source/platform/mhtml

It no longer depends on anything in core besides itself. Rename to mhtml since
we only support one archive format at this point.

BUG=none

Review URL: https://codereview.chromium.org/68933014

git-svn-id: svn://svn.chromium.org/blink/trunk@161980 bbb929c8-8fbe-4397-9dbb-9b2b20218538