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}

media/formats/BUILD.gn

@@ -20,6 +20,8 @@ source_set("formats") {
     "ac3/ac3_util.h",
     "common/offset_byte_queue.cc",
     "common/offset_byte_queue.h",
+    "mp4/bitstream_converter.cc",
+    "mp4/bitstream_converter.h",
     "mp4/box_definitions.cc",
     "mp4/box_definitions.h",
     "mp4/box_reader.cc",
@@ -87,8 +89,6 @@ source_set("formats") {
       "mp4/aac.h",
       "mp4/avc.cc",
       "mp4/avc.h",
-      "mp4/bitstream_converter.cc",
-      "mp4/bitstream_converter.h",
       "mpeg/adts_constants.cc",
       "mpeg/adts_constants.h",
       "mpeg/adts_stream_parser.cc",