Reland of Add a flags field to video timing extension. (patchset #1 id:1 of https://codereview.webrtc.org/2995953002/ )

Reason for revert:
Create reland CL to add fix to.

Original issue's description:
> Revert of Add a flags field to video timing extension. (patchset #15 id:280001 of https://codereview.webrtc.org/3000753002/ )
>
> Reason for revert:
> Speculative revet for breaking remoting_unittests in fyi bots.
> https://build.chromium.org/p/chromium.webrtc.fyi/waterfall?builder=Win7%20Tester
>
> Original issue's description:
> > Add a flags field to video timing extension.
> >
> > The rtp header extension for video timing shuold have an additional
> > field for signaling metadata, such as what triggered the extension for
> > this particular frame. This will allow separating frames select because
> > of outlier sizes from regular frames, for more accurate stats.
> >
> > This implementation is backwards compatible in that it can read video
> > timing extensions without the new flag field, but it always sends with
> > it included.
> >
> > BUG=webrtc:7594
> >
> > Review-Url: https://codereview.webrtc.org/3000753002
> > Cr-Commit-Position: refs/heads/master@{#19353}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/cf5d485e147f7d7b3081692f101e496ce9e1d257
>
> TBR=danilchap@webrtc.org,kthelgason@webrtc.org,stefan@webrtc.org,sprang@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:7594
>
> Review-Url: https://codereview.webrtc.org/2995953002
> Cr-Commit-Position: refs/heads/master@{#19360}
> Committed: https://chromium.googlesource.com/external/webrtc/+/f0f7378b059501bb2bc5d006bf0f43546e47328f

TBR=danilchap@webrtc.org,kthelgason@webrtc.org,stefan@webrtc.org,emircan@google.com
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:7594

Review-Url: https://codereview.webrtc.org/2996153002
Cr-Commit-Position: refs/heads/master@{#19405}
diff --git a/webrtc/modules/video_coding/frame_object.cc b/webrtc/modules/video_coding/frame_object.cc
index 1d858fc..86bcffb 100644
--- a/webrtc/modules/video_coding/frame_object.cc
+++ b/webrtc/modules/video_coding/frame_object.cc
@@ -32,6 +32,7 @@
     : packet_buffer_(packet_buffer),
       first_seq_num_(first_seq_num),
       last_seq_num_(last_seq_num),
+      timestamp_(0),
       received_time_(received_time),
       times_nacked_(times_nacked) {
   VCMPacket* first_packet = packet_buffer_->GetPacket(first_seq_num);
@@ -113,10 +114,10 @@
   rotation_ = last_packet->video_header.rotation;
   _rotation_set = true;
   content_type_ = last_packet->video_header.content_type;
-  if (last_packet->video_header.video_timing.is_timing_frame) {
+  if (last_packet->video_header.video_timing.flags !=
+      TimingFrameFlags::kInvalid) {
     // ntp_time_ms_ may be -1 if not estimated yet. This is not a problem,
     // as this will be dealt with at the time of reporting.
-    timing_.is_timing_frame = true;
     timing_.encode_start_ms =
         ntp_time_ms_ +
         last_packet->video_header.video_timing.encode_start_delta_ms;
@@ -138,9 +139,8 @@
 
     timing_.receive_start_ms = first_packet->receive_time_ms;
     timing_.receive_finish_ms = last_packet->receive_time_ms;
-  } else {
-    timing_.is_timing_frame = false;
   }
+  timing_.flags = last_packet->video_header.video_timing.flags;
 }
 
 RtpFrameObject::~RtpFrameObject() {