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_buffer.cc b/webrtc/modules/video_coding/frame_buffer.cc
index 41c7fc8..f67210f 100644
--- a/webrtc/modules/video_coding/frame_buffer.cc
+++ b/webrtc/modules/video_coding/frame_buffer.cc
@@ -164,7 +164,8 @@
     rotation_ = packet.video_header.rotation;
     _rotation_set = true;
     content_type_ = packet.video_header.content_type;
-    if (packet.video_header.video_timing.flags != TimingFrameFlags::kInvalid) {
+    if (packet.video_header.video_timing.is_timing_frame) {
+      timing_.is_timing_frame = true;
       timing_.encode_start_ms =
           ntp_time_ms_ + packet.video_header.video_timing.encode_start_delta_ms;
       timing_.encode_finish_ms =
@@ -181,8 +182,9 @@
       timing_.network2_timestamp_ms =
           ntp_time_ms_ +
           packet.video_header.video_timing.network2_timstamp_delta_ms;
+    } else {
+      timing_.is_timing_frame = false;
     }
-    timing_.flags = packet.video_header.video_timing.flags;
   }
 
   if (packet.is_first_packet_in_frame) {