Cast: removed is_sender from CastLoggingConfig / LoggingImpl.

With the subscriber model, CastLoggingConfig / LoggingImpl no longer need to
keep track of this.

I am keeping the two helper functions GetDefaultCastSenderLoggingConfig() / GetDefaultCastReceiverLoggingConfig(), in case their configs differ in the future.

Review URL: https://codereview.chromium.org/159623003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250634 0039d316-1c4b-4281-b951-d872f2087c98
LKGR w/ DEPS up to revision 250634