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

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