Replacing "using std::string" with fully qualified namespace on
each occurance of string in sample_muxer_metadata.cc and
webvttparser.cc.
Also fixing a formatting nit in mkvwriteeofreader.cpp
Change-Id: Icf713f9e489fbdc9af14e83d0cb7ba2e89e65ab4
Changing the IMkvReadableWriter interface to IMkvWriteEOFReader.
Also changing the default implementation.
Change-Id: Id37ffd7ef0af2ff7a392fb4fb0b1b134664ab20f