HAVE(MMAP), HAVE(VASPRINTF) and HAVE(DISPATCH_H) guards are no longer used, we should remove them.

Review URL: https://codereview.chromium.org/165183003

git-svn-id: svn://svn.chromium.org/blink/trunk@167154 bbb929c8-8fbe-4397-9dbb-9b2b20218538