Reland of Fix off-by-one bugs in video_coding::PacketBuffer when the buffer is filled with a single frame. (patchset #1 id:1 of https://codereview.chromium.org/2990183002/ )

Reason for revert:
Revert to create fix CL.

Original issue's description:
> Revert of Fix off-by-one bugs in video_coding::PacketBuffer when the buffer is filled with a single frame. (patchset #5 id:80001 of https://codereview.chromium.org/2993513002/ )
>
> Reason for revert:
> Break performance bots.
>
> Original issue's description:
> > Fix off-by-one bugs in video_coding::PacketBuffer when the buffer is filled with a single frame.
> >
> > BUG=webrtc:8028
> >
> > Review-Url: https://codereview.webrtc.org/2993513002
> > Cr-Commit-Position: refs/heads/master@{#19209}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/ee13e8919c20de5860a510e91fac71fd5a7e9b8d
>
> TBR=stefan@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:8028
>
> Review-Url: https://codereview.webrtc.org/2990183002
> Cr-Commit-Position: refs/heads/master@{#19211}
> Committed: https://chromium.googlesource.com/external/webrtc/+/c18f1d7c9432a2bf7d112820eb2b1dd2dbe2ba4b

TBR=stefan@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:8028
TBR=stefan@webrtc.org

Review-Url: https://codereview.webrtc.org/2989313003
Cr-Commit-Position: refs/heads/master@{#19249}
diff --git a/webrtc/modules/video_coding/frame_object.cc b/webrtc/modules/video_coding/frame_object.cc
index 4c4537c..25d0e29 100644
--- a/webrtc/modules/video_coding/frame_object.cc
+++ b/webrtc/modules/video_coding/frame_object.cc
@@ -93,7 +93,8 @@
     frame_type_ = first_packet->frameType;
   }
 
-  GetBitstream(_buffer);
+  bool bitstream_copied = GetBitstream(_buffer);
+  RTC_DCHECK(bitstream_copied);
   _encodedWidth = first_packet->width;
   _encodedHeight = first_packet->height;