Add VideoTimingExtension to kFecOrPaddingExtensionSizes.

As of https://webrtc-review.googlesource.com/c/src/+/158899, FEC may be
used on packets with VideoTimingExtension.  This may result in creation
of FEC packets that exceed the maximum configured RTP packet size.

This problem occurs most frequently with datagram transports that define a
smaller maximum packet size.

Bug: webrtc:9719
Change-Id: I842216a6696a695f0a3f01a221e538605fc5b9bd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/161557
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Bjorn Mellem <mellem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30045}

modules/rtp_rtcp/source/rtp_sender.cc

@@ -63,6 +63,7 @@ constexpr RtpExtensionSize kFecOrPaddingExtensionSizes[] = {
     CreateExtensionSize<TransportSequenceNumber>(),
     CreateExtensionSize<PlayoutDelayLimits>(),
     CreateMaxExtensionSize<RtpMid>(),
+    CreateExtensionSize<VideoTimingExtension>(),
 };
 
 // Size info for header extensions that might be used in video packets.