Chapters::*::Parse: replace pos asserts w/checks
Chapters::*::Parse: replace pos asserts w/checks

replace the common patterns assert(pos <= stop) / assert(pos == stop)
with error checks

BUG=23431751

Change-Id: I3425f6083456b5ab4f40497e3a192569b494dfa9

zofuthan/libwebm,felipebetancur/libwebm,webmproject/libwebm,zofuthan/libwebm,webmproject/libwebm,felipebetancur/libwebm,webmproject/libwebm