Seems like there's a USE_PROPRIETARY_CODECS #define lurking around MP4 where there shouldn't be anymore.

Fix out for review
https://chromium-review.googlesource.com/c/chromium/src/+/819978

Surprised the bots didn't catch this. My non-proprietary build compiles fine, so something about the ubsan flags surfaced it.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/561193ab845c0eb9457ce7104b3b2f41565269d7

commit 561193ab845c0eb9457ce7104b3b2f41565269d7
Author: Chris Cunningham <chcunningham@chromium.org>
Date: Mon Dec 11 22:56:06 2017

Include bitstream converters to fix UBSan vptr error

The VideoSampleEntry box includes a scoped_refptr<BitstreamConverter>
regardless always, but we were only building the BitstreamConverter cc/h
files when proprietary_codecs=true. This CL includes the
BitstreamConverter interface defintion in all builds.

Proprietary bitsream conversion logic (e.g. AVCBitstreamConvter) is
still only included if proprietary_codecs = true.

BUG:  793702 
Change-Id: I8720b6547ca2e0af47d64ae594e86850da0e84b5
Reviewed-on: https://chromium-review.googlesource.com/819978
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523242}
[modify] https://crrev.com/561193ab845c0eb9457ce7104b3b2f41565269d7/media/formats/BUILD.gn