Refactor string internalization.

Remove duplicate code in StringTable and Heap. Instead of having many InternalizeXXXString methods, add a general InternalizeStringWithKey that takes a string table key.

BUG=
R=dcarney@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/140953002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18657 ce2b1a6d-e550-0410-aec6-3dcde31c8c00