Bug 287149

Functions x509Serialize() and privateKeySerialize() in Source/WebCore/Modules/mediastream/gstreamer/GStreamerWebRTCUtils.cpp use an intermedia Vector<char> as a temporary buffer to read back the values that have been serialized via BIO_write_*, then create a WTF::String that will contain a copy of the buffer contents. Instead of doing this, we can use BIO_get_mem_data() to pass the underlying buffer used by the BIO functions to create the String, and avoid one copy into the Vector<char> buffer.