Add visibility attributes to LLVMFuzzerInitialize on Mac in JS parser proto fuzzer.

That prevents linker on Mac from dead-stripping the function, as it's actually used
but is being called indirectly using dlsym().

Bug: 837477
Change-Id: Ic02066132cba3e1f9deb977219b4f3ec13207991
Reviewed-on: https://chromium-review.googlesource.com/1050505Reviewed-by: Jonathan Metzman <metzman@chromium.org>
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556844}

testing/libfuzzer/fuzzers/javascript_parser_proto_fuzzer.cc

@@ -27,7 +27,14 @@ std::string protobuf_to_string(
   return source;
 }
 
-extern "C" int LLVMFuzzerInitialize(int* argc, char*** argv) {
+// Explicitly specify some attributes to avoid issues with the linker dead-
+// stripping the following function on macOS, as it is not called directly
+// by fuzz target. LibFuzzer runtime uses dlsym() to resolve that function.
+#if V8_OS_MACOSX
+__attribute__((used)) __attribute__((visibility("default")))
+#endif  // V8_OS_MACOSX
+extern "C" int
+LLVMFuzzerInitialize(int* argc, char*** argv) {
   v8::V8::InitializeICUDefaultLocation((*argv)[0]);
   v8::V8::InitializeExternalStartupData((*argv)[0]);
   v8::V8::SetFlagsFromCommandLine(argc, *argv, true);