Reland "Removed default Isolate."

Changes compared to original CL: Until the Serializer/Isolate chaos is
cleaned up, we can't initalize/dispose cctests in a tidy way, see
cctest.cc.

BUG=359977
LOG=y
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/267383002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21205 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Use ReplaceCode instead of set_code in debug.cc.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/274043002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21206 ce2b1a6d-e550-0410-aec6-3dcde31c8c00