[code coverage] Pass target_os to wrapper on Mac.

Bug: 1141727
Change-Id: I763275bb48fd368cbb7ce7bfe5f9979c81201042
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2560835
Auto-Submit: Sajjad Mirza <sajjadm@chromium.org>
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831912}

build/toolchain/mac/BUILD.gn

@@ -157,7 +157,7 @@ template("mac_toolchain") {
           rebase_path("//build/toolchain/clang_code_coverage_wrapper.py",
                       root_build_dir) + " --files-to-instrument=" +
           rebase_path(toolchain_coverage_instrumentation_input_file,
-                      root_build_dir)
+                      root_build_dir) + " --target-os=" + target_os
       cc = _coverage_wrapper + " ${cc}"
       cxx = _coverage_wrapper + " ${cxx}"
     }