Having that dependency require user of RtpPacket to ensure RtpHeaderExtensionMap always outlive packet and that RtpPacket's access to RtpHeaderExtensionMap is properly syncrhonized. Dropping this dependencies make use of RtpPacket less error-prone. BUG=webrtc:5261 Review-Url: https://codereview.webrtc.org/2576653003 Cr-Commit-Position: refs/heads/master@{#15653}