Find Period element under MPD
Find Period element under MPD

It assumed that the first element is Period but it could be a BaseURL
which caused a DCHECK crash.

Change-Id: I75bdafdd312bfbcdf81cc76b44f8e2d0f1542976

nevil/edash-packager,leandromoreira/edash-packager,leandromoreira/edash-packager,leandromoreira/edash-packager,nevil/edash-packager,nevil/edash-packager,leandromoreira/edash-packager,leandromoreira/edash-packager