Enable the "unused variable" warning on Windows.

- Break out direct_show_base_classes to its own gyp file to have it
  treated as third party code.
- Fix the resulting warnings (courtesy of Tommi).

BUG=
TEST=build on Windows (vie_auto_test currently failing at HEAD)

Review URL: https://webrtc-codereview.appspot.com/489001

git-svn-id: http://webrtc.googlecode.com/svn/trunk@2000 4adac7df-926f-26a2-2b94-8c16560cd09d
24 files changed