message: Clarification of RtpPacket constructor in comment.
  See also b/175210069 for more context.
  Cr-Commit-Position: refs/heads/master@{#32874}
diff: @@ -30,6 +30,8 @@ class RtpPacket {
  // packet creating and used if available in Parse function.
  // Adding and getting extensions will fail until |extensions| is
  // provided via constructor or IdentifyExtensions function.
  + // |*extensions| is only accessed during construction; the pointer is not
  + // stored.
  RtpPacket();
  explicit RtpPacket(const ExtensionManager* extensions);
  RtpPacket(const RtpPacket&);
  