get rid of Mac visibility warnings

Command line builds and xcode projects on the Mac generate hundreds of warnings complaining about mismatched visibility between files. While there's no way to suppress the warnings, the visibility can be made the same across the board.

[epoger, 2012-05-30 19:46:31]

Rob, is http://code.google.com/p/skia/issues/detail?id=555 ('"bad codegen,
pointer diff in SkRefCnt::SkRefCnt()" when building "tests" app on Mac') still
affecting the build for you?  

Does this CL make a difference?

[robertphillips, 2012-05-30 19:55:05]

The patch we applied in http://codereview.appspot.com/5992048/ got the Mac port
compiling for me. I hope this change will get rid of the remaining asserts.

[robertphillips, 2012-05-30 20:03:38]

So LGTM.

[epoger, 2012-05-30 20:08:23]

LGTM