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}

webrtc/modules/video_coding/generic_decoder.cc

diff --git a/webrtc/modules/video_coding/generic_decoder.cc b/webrtc/modules/video_coding/generic_decoder.cc
index cc14794..3b2a620 100644
--- a/webrtc/modules/video_coding/generic_decoder.cc
+++ b/webrtc/modules/video_coding/generic_decoder.cc
@@ -92,7 +92,7 @@
                            frameInfo->renderTimeMs);
 
   // Report timing information.
-  if (frameInfo->timing.is_timing_frame) {
+  if (frameInfo->timing.flags != TimingFrameFlags::kInvalid) {
     int64_t capture_time_ms = decodedImage.ntp_time_ms() - ntp_offset_;
     // Convert remote timestamps to local time from ntp timestamps.
     frameInfo->timing.encode_start_ms -= ntp_offset_;
@@ -137,6 +137,7 @@
     timing_frame_info.decode_finish_ms = now_ms;
     timing_frame_info.render_time_ms = frameInfo->renderTimeMs;
     timing_frame_info.rtp_timestamp = decodedImage.timestamp();
+    timing_frame_info.flags = frameInfo->timing.flags;
 
     _timing->SetTimingFrameInfo(timing_frame_info);
   }