[Author: grobbins]
[Author: grobbins]

Add ARCMacros for use in code that may compile with or without ARC enabled.

R=thomasvl,dmaclach
APPROVED=thomasvl

jonreid/google-toolbox-for-mac,qwertme/google-toolbox-for-mac,jonreid/google-toolbox-for-mac,jonreid/google-toolbox-for-mac,qwertme/google-toolbox-for-mac,qwertme/google-toolbox-for-mac