Cast: Refactored SendPacket to send a burst of packets;
instead of one packet per callback. The implementer should
implement the new virtual function SendPackets that send
multiple packets as fast as possible. The purpose is to
minimize the number of IPC messages.
No changes to the actual pacing logic. 
In the process also cleaned up the usage of std::set<uint16>
even though the define PacketIdSet existed.

Review URL: https://codereview.chromium.org/26618002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228088 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228088
Finishing API error processing, and cleanup.
The cleanup includes:
* Getting rid of callback param to tasks
* Getting rid of debugSetStepName

BUG=164227
TEST=No

Review URL: https://codereview.chromium.org/25114002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228089 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228089