base: add thread pool for CPU bound tasks

This is a very lightweight implementation building heavily on top of
ThreadTaskRunner and simply adding a queue of tasks which allows load
balancing across threads and a stack tracking the threads which are
immediately available for scheduling.

Change-Id: I8a7fa3fde327e16fd7aa9f6735cbe4db95b65959
diff --git a/gn/perfetto_unittests.gni b/gn/perfetto_unittests.gni
index 3af1962..76594fe 100644
--- a/gn/perfetto_unittests.gni
+++ b/gn/perfetto_unittests.gni
@@ -18,6 +18,7 @@
   "gn:default_deps",
   "gn:gtest_main",
   "src/base:unittests",
+  "src/base/threading:unittests",
   "src/protozero:unittests",
   "src/tracing/core:unittests",
   "src/tracing:unittests",