Fix Vulkan fuzzer on mac_libfuzzer_chrome_asan

This commit fixes two problems:
1) It includes testing/libfuzzer/libfuzzer_exports.h which exports
   the LLVMFuzzerInitialize symbol. On Mac, not doing so causes it
   to be removed by the linker.
2) It makes it so that dawn_use_swiftshader is enabled by default
   when fuzzing, and enabling swiftshader also enables the Vulkan
   backend.

Bug: chromium:1042426
Change-Id: Ic0ae0d59c91476b0079148f33a4b1ebd1cc6743c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/15462
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>

src/fuzzers/DawnWireServerAndVulkanBackendFuzzer.cpp

@@ -15,6 +15,7 @@
 #include "DawnWireServerFuzzer.h"
 
 #include "dawn_native/DawnNative.h"
+#include "testing/libfuzzer/libfuzzer_exports.h"
 
 extern "C" int LLVMFuzzerInitialize(int* argc, char*** argv) {
     return DawnWireServerFuzzer::Initialize(argc, argv);