Make peeking before the head of OffsetByteQueue a hard error.

media/mp4/offset_byte_queue.cc

Index: media/mp4/offset_byte_queue.cc
diff --git a/media/mp4/offset_byte_queue.cc b/media/mp4/offset_byte_queue.cc
index 862d9370b20b58ffdee8f874c5a4ba2b6c574e44..a530150899bce18a9ec56c9c8d03aa361943864e 100644
--- a/media/mp4/offset_byte_queue.cc
+++ b/media/mp4/offset_byte_queue.cc
@@ -37,6 +37,7 @@ void OffsetByteQueue::Pop(int count) {
 }
 
 void OffsetByteQueue::PeekAt(int64 offset, const uint8** buf, int* size) {
+  DCHECK(offset >= head());

[acolwell GONE FROM CHROMIUM, 2012/07/30 20:49:04]

Is this a result of a coding error, or can unexpected or corrupt files trigger
this? If it is the former then I'm fine with the DCHECK. If it is the later then
I'd prefer the original code since this situation should allow us to signal an
error w/o crashing.

[strobe_, 2012/07/31 18:34:38]

As of now, it can only be the result of a programming error. (Formerly, it could
have been the result of a corrupt file, but recent changes eliminate that.)

   if (offset < head() || offset >= tail()) {
     *buf = NULL;
     *size = 0;