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}
diff --git a/webrtc/modules/video_coding/frame_object.cc b/webrtc/modules/video_coding/frame_object.cc
index 86bcffb..1d858fc 100644
--- a/webrtc/modules/video_coding/frame_object.cc
+++ b/webrtc/modules/video_coding/frame_object.cc
@@ -32,7 +32,6 @@
     : 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);
@@ -114,10 +113,10 @@
   rotation_ = last_packet->video_header.rotation;
   _rotation_set = true;
   content_type_ = last_packet->video_header.content_type;
-  if (last_packet->video_header.video_timing.flags !=
-      TimingFrameFlags::kInvalid) {
+  if (last_packet->video_header.video_timing.is_timing_frame) {
     // 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;
@@ -139,8 +138,9 @@
 
     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() {