Function calls that may return a failure must use the result. Enforce
this by adding missing MUST_USE_RESULT to their declarations.
Review URL: https://chromiumcodereview.appspot.com/9421032

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10748 ce2b1a6d-e550-0410-aec6-3dcde31c8c00