Migrate to OS_MAC and OS_APPLE in third_party

Bug: 1105907
Change-Id: I1784cf675b6bc2bcf7921e4b7535c9ea9cd9e2d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2324897Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792579}

third_party/brotli/common/platform.h

@@ -33,7 +33,7 @@
 #include <endian.h>
 #elif defined(OS_FREEBSD)
 #include <machine/endian.h>
-#elif defined(OS_MACOSX)
+#elif defined(OS_APPLE)
 #include <machine/endian.h>
 /* Let's try and follow the Linux convention */
 #define BROTLI_X_BYTE_ORDER BYTE_ORDER