Add support for fragmentation in RtcpPacket.

If the buffer becomes full an OnPacketReady callback will be used to
send the packets created so far. On success the buffer can be reused.
The same callback will be called when the last packet has beed created.

Also made some changes to RawPacket. Buffer will now be heap-allocated
rather than (potentially) stack-allocated, but on the plus side it can
now be allocted with variable size and also avoids one memcpy.

BUG=

patch from issue 56429004 at patchset 160001 (http://crrev.com/56429004#ps160001)

R=asapersson@webrtc.org

Review URL: https://codereview.webrtc.org/1165113002

Cr-Commit-Position: refs/heads/master@{#9390}
5 files changed