For WebM contents, it is possible that we may receive some NewSampleEvent
before receiving ParserInitEvent. This is because init event is fired
after analyzing a video block which could come after an audio block.
Also modified Flush() function to return a bool to indicate whether the
the flush is successful and whether the samples are handled correctly.
And added macro to ensure Flush() and Parse() results are handled.
Fixes#71
Change-Id: I2294d6f529f54e4578344916559bb1bc116c745a
Kongqun Yang
Rintaro Kuroiwa