Fix Windows debug compile of overrides/ logging.

Compile error detected when trying to roll to chromium. Adding a cast
of base::PlatformThread::CurrentId() to base::subtle::Atomic32 to match
types in DCHECK_EQ().

See logging.cc error in:
http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_compile_dbg/builds/19944/steps/compile%20%28with%20patch%29/logs/stdio

R=mflodman@webrtc.org, perkj@webrtc.org
TBR=henrike@webrtc.org
BUG=

Review URL: https://webrtc-codereview.appspot.com/17529004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6173 4adac7df-926f-26a2-2b94-8c16560cd09d