lint cleanup for convert RGB24ToI420

RGB24, RAW, RGB565, ARGB1555 and ARGB4444 have conditional
2 pass versus direct path.  2 pass method requires a buffer that
is conditionally allocated.  ifdef's were confusing lint.
simplifed ifdefs to clean up lint warning

BUG=libyuv:692
TEST=lint source/convert.cc

Change-Id: If868718af30b48824a5e3d28f0d7d01d4609ad55
Reviewed-on: https://chromium-review.googlesource.com/451552Reviewed-by: Henrik Kjellander <kjellander@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@google.com>

README.chromium

 Name: libyuv
 URL: http://code.google.com/p/libyuv/
-Version: 1648
+Version: 1649
 License: BSD
 License File: LICENSE
 

include/libyuv/version.h

@@ -11,6 +11,6 @@
 #ifndef INCLUDE_LIBYUV_VERSION_H_
 #define INCLUDE_LIBYUV_VERSION_H_
 
-#define LIBYUV_VERSION 1648
+#define LIBYUV_VERSION 1649
 
 #endif  // INCLUDE_LIBYUV_VERSION_H_