Add support for fragmentation in RtcpPacket.

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

Committed: https://crrev.com/c1b9d4e686c184e4b1779d442d447128477d3b8b
Cr-Commit-Position: refs/heads/master@{#9390}

[sprang_webrtc, 2015-06-05 13:24:40]

sprang@webrtc.org changed reviewers:
+ asapersson@webrtc.org

[sprang_webrtc, 2015-06-05 13:24:40]

Patched CL over to new rietveld.

[åsapersson, 2015-06-05 14:09:59]

https://codereview.webrtc.org/1165113002/diff/20001/webrtc/modules/rtp_rtcp/s...
File webrtc/modules/rtp_rtcp/source/rtcp_packet_unittest.cc (right):

https://codereview.webrtc.org/1165113002/diff/20001/webrtc/modules/rtp_rtcp/s...
webrtc/modules/rtp_rtcp/source/rtcp_packet_unittest.cc:452: const size_t
buffer_size = 12 + (3 * 4);  // Fits common header + 3 nack items
buffer_size->kBufferSize

[åsapersson, 2015-06-05 14:57:55]

On 2015/06/05 14:09:59, asapersson wrote:
>
https://codereview.webrtc.org/1165113002/diff/20001/webrtc/modules/rtp_rtcp/s...
> File webrtc/modules/rtp_rtcp/source/rtcp_packet_unittest.cc (right):
> 
>
https://codereview.webrtc.org/1165113002/diff/20001/webrtc/modules/rtp_rtcp/s...
> webrtc/modules/rtp_rtcp/source/rtcp_packet_unittest.cc:452: const size_t
> buffer_size = 12 + (3 * 4);  // Fits common header + 3 nack items
> buffer_size->kBufferSize

lgtm (see one comment)

[sprang_webrtc, 2015-06-08 07:54:30]

Committed patchset #3 (id:40001) manually as
c1b9d4e686c184e4b1779d442d447128477d3b8b (presubmit successful).

[commit-bot: I haz the power, 2015-06-08 07:54:38]

Patchset 3 (id:??) landed as
https://crrev.com/c1b9d4e686c184e4b1779d442d447128477d3b8b
Cr-Commit-Position: refs/heads/master@{#9390}

[sprang_webrtc, 2015-06-11 08:22:01]

https://codereview.webrtc.org/1165113002/diff/20001/webrtc/modules/rtp_rtcp/s...
File webrtc/modules/rtp_rtcp/source/rtcp_packet_unittest.cc (right):

https://codereview.webrtc.org/1165113002/diff/20001/webrtc/modules/rtp_rtcp/s...
webrtc/modules/rtp_rtcp/source/rtcp_packet_unittest.cc:452: const size_t
buffer_size = 12 + (3 * 4);  // Fits common header + 3 nack items
On 2015/06/05 14:09:59, asapersson wrote:
> buffer_size->kBufferSize

Done.