If we swap its arguments, SkTaskGroup::batch() _is_ sk_parallel_for.

Why have two names if we can get away with one?

This kills off sk_parallel_for_thread_count(), which was only used to avoid forcing a deadlock in OncePtrTest on multicore machines in singlethreaded mode... a really niche use case.  Instead just don't explicitly force a race.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1552093002

Review URL: https://codereview.chromium.org/1552093002
diff --git a/tests/PathOpsThreadedCommon.cpp b/tests/PathOpsThreadedCommon.cpp
index 342b560..c9a06f0 100644
--- a/tests/PathOpsThreadedCommon.cpp
+++ b/tests/PathOpsThreadedCommon.cpp
@@ -16,7 +16,7 @@
 }
 
 void PathOpsThreadedTestRunner::render() {
-    sk_parallel_for(fRunnables.count(), [&](int i) {
+    SkTaskGroup().batch(fRunnables.count(), [&](int i) {
         fRunnables[i]->run();
     });
 }