Explicitly initialize MarkCompactCollector and ExternalStringTable

BUG=none
R=dcarney@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/143063003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18689 ce2b1a6d-e550-0410-aec6-3dcde31c8c00