Fix display of WTF::Vectors in gdb.
Fix display of WTF::Vectors in gdb.

WTF::Vector now inherits from VectorBufferBase instead of containing it as an
m_buffer member.

R=tony@chromium.org

Review URL: https://codereview.chromium.org/18337009

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@153715 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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