Cast: move net->transport

 - Moving files to the transport folder.
 - Adding transport namespace.
 - Consolidating all gyp's into cast_transport.gyp.
 - Misc clean up

This cl should be patched on top of https://codereview.chromium.org/102413014/

BUG=327482

Review URL: https://codereview.chromium.org/100823015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242557 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242557