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}