Fix integer cast warnings in Cast code

Fixing several spots in media/cast that caused compiler warnings.

Review URL: https://codereview.chromium.org/25671003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226697 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226697