Migrate to OS_MAC and OS_APPLE in testing

Bug: 1105907
Change-Id: I1af1d0a3e595d1bc1dc6ab380743834a71b03777
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2324214
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792844}

testing/libfuzzer/libfuzzer_exports.h

@@ -13,7 +13,7 @@
 // this header in the fuzzer's implementation file will ensure that all the
 // symbols are kept and exported.
 
-#if defined(OS_MACOSX)
+#if defined(OS_MAC)
 #define EXPORT_FUZZER_FUNCTION \
   __attribute__((used)) __attribute__((visibility("default")))
 #else