Rename webrtc/base's IS_ALIGNED macro to RTC_IS_ALIGNED to avoid conflict between webrtc/base/basictypes.h and third_party/.../vpx_codec.h.

BUG=3380
R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/17579005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6215 4adac7df-926f-26a2-2b94-8c16560cd09d