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>
2 files changed