webmdec: quiet -Wshorten-64-to-32 warning

track->GetNumber() will fit in an int in well-behaved files

cherry-picked from libvpx:
c79665d0a webmdec: quiet -Wshorten-64-to-32 warning

Change-Id: I344a7dd05d04daf3df2d67358ea69f8014a03a5b
1 file changed