Reland "Pass-through batch-limit, jobs & single-process options and tag OS."

This is a reland of b377a02f46c52b2119440e171ae25a4af1d87f05
Original change's description:
> Pass-through batch-limit, jobs & single-process options and tag OS.
> 
> Adds pass-through of some command-line options useful for debugging:
> --test-launcher-batch-limit to control tests-per-process.
> --test-launcher-jobs to control # of parallel test processes.
> --single-process-tests to run all tests in a single process.
> 
> Also adds the OS_FUCHSIA tag to the test results output.
> 
> Change-Id: I4ce99f2a33536ddd9fae4fcb6bf93eab6f38a612
> Reviewed-on: https://chromium-review.googlesource.com/564738
> Commit-Queue: Wez <wez@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#485516}

Change-Id: Ib81b640818e6b6751135e4780f31135e15e6bc23
Reviewed-on: https://chromium-review.googlesource.com/566899
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485811}

CrOS-Libchrome-Original-Commit: 0615b2a9a1ea9484a60e3c7cbb81bcd5dd7b3148
diff --git a/base/test/launcher/test_launcher.cc b/base/test/launcher/test_launcher.cc
index cd64bda..8959e1e 100644
--- a/base/test/launcher/test_launcher.cc
+++ b/base/test/launcher/test_launcher.cc
@@ -900,6 +900,10 @@
   results_tracker_.AddGlobalTag("OS_FREEBSD");
 #endif
 
+#if defined(OS_FUCHSIA)
+  results_tracker_.AddGlobalTag("OS_FUCHSIA");
+#endif
+
 #if defined(OS_IOS)
   results_tracker_.AddGlobalTag("OS_IOS");
 #endif