Dummy implementation of CastTransportSender

This change is an intermediate step for moving the cast transport module
from renderer to the broewser. We're implementing a real
CastTransportSender that uses IPC to send encoded video frames to the
browser process. In the mean time we want to re-enable tests for
cast.streaming extension APIS.

Another change made is the removal of P2P socket in cast code. P2P
socket is now obsolete in Cast because we are not sending packets in
the renderer any more.

CastStreamingApiTest nows runs without crashing.

TBR=kalman for a trivial change in
chrome/browser/extensions/cast_streaming_apitest.cc

BUG=301920, 335591

Review URL: https://codereview.chromium.org/144423008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247175 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247175