Revert "Allow arbitrary names for weak references in heap snapshots."

This reverts commit r18838 for breaking build with clang. Errors are:

../../src/heap-snapshot-generator.cc:1217:53: error: empty macro arguments were standardized in C99 [-Werror,-pedantic]
    EXTRACT_CONTEXT_FIELD(OPTIMIZED_FUNCTIONS_LIST, , optimized_functions_list);
../../src/heap-snapshot-generator.cc:1218:48: error: empty macro arguments were standardized in C99 [-Werror,-pedantic]
    EXTRACT_CONTEXT_FIELD(OPTIMIZED_CODE_LIST, , optimized_code_list);
../../src/heap-snapshot-generator.cc:1219:50: error: empty macro arguments were standardized in C99 [-Werror,-pedantic]
    EXTRACT_CONTEXT_FIELD(DEOPTIMIZED_CODE_LIST, , deoptimized_code_list);
../../src/heap-snapshot-generator.cc:1220:46: error: empty macro arguments were standardized in C99 [-Werror,-pedantic]
    EXTRACT_CONTEXT_FIELD(NEXT_CONTEXT_LINK, , next_context_link);

TBR=alph@chromium.org

Review URL: https://codereview.chromium.org/145583003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18844 ce2b1a6d-e550-0410-aec6-3dcde31c8c00