Add a libfuzzer for RtpHeaderParser.

NOTRY=true

Review-Url: https://codereview.webrtc.org/2062103002
Cr-Commit-Position: refs/heads/master@{#13271}

webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h

@@ -68,6 +68,7 @@ enum RTPExtensionType {
   kRtpExtensionVideoRotation,
   kRtpExtensionTransportSequenceNumber,
   kRtpExtensionPlayoutDelay,
+  kRtpExtensionNumberOfExtensions,
 };
 
 enum RTCPAppSubTypes { kAppSubtypeBwe = 0x00 };