Make flag parsing work in GN-built iOS nanobench.

Just like DM.

CQ_INCLUDE_TRYBOTS=skia.primary:Test-iOS-Clang-iPadMini4-GPU-GX6450-Arm7-Debug,Build-Mac-Clang-arm64-Debug-GN_iOS

Change-Id: I4af3fa1813e3b7ee48407096e91373b5fee569c7
Reviewed-on: https://skia-review.googlesource.com/7824
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

bench/nanobench.cpp

@@ -1361,7 +1361,7 @@ int nanobench_main() {
     return 0;
 }
 
-#if !defined SK_BUILD_FOR_IOS
+#if !defined(SK_BUILD_FOR_IOS) || defined(NANOBENCH_DEFINE_MAIN)
 int main(int argc, char** argv) {
     SkCommandLineFlags::Parse(argc, argv);
     return nanobench_main();

gn/BUILD.gn

@@ -212,7 +212,7 @@ config("default") {
     # our clever hack to give each tool back its own main().
     cflags += [
       "-DDM_DEFINE_MAIN",
-      "-Dnanobench_main=main",
+      "-DNANOBENCH_DEFINE_MAIN",
       "-Dtool_main=main",
       "-Dtest_main=main",
     ]