Revert "Replace HeapNumber as doublebox with an explicit MutableHeapNumber."

This reverts commit r22082 for breaking arm64 build.

TBR=verwaest@chromium.org

Review URL: https://codereview.chromium.org/360023003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22083 ce2b1a6d-e550-0410-aec6-3dcde31c8c00