Replace further questionable HashMap::add usages in bindings

BUG=390928
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/411273002

git-svn-id: svn://svn.chromium.org/blink/trunk@178823 bbb929c8-8fbe-4397-9dbb-9b2b20218538
    String takeWireString()
    {
        COMPILE_ASSERT(sizeof(BufferValueType) == 2, BufferValueTypeIsTwoBytes);
        fillHole();
        String data = String(m_buffer.data(), m_buffer.size());
        data.impl()->truncateAssumingIsolated((m_position + 1) / sizeof(BufferValueType));
        return data;
    }

    String takeWireString()
    {
        COMPILE_ASSERT(sizeof(BufferValueType) == 2, BufferValueTypeIsTwoBytes);
        fillHole();
        String data = String(m_buffer.data(), m_buffer.size());
        data.impl()->truncateAssumingIsolated((m_position + 1) / sizeof(BufferValueType));
        return data;
    }
