Update GC stats, ThreadPool, Timer and ScopedTimer to use monotonic time.

runtime/vm/thread_pool_test.cc

Index: runtime/vm/thread_pool_test.cc
diff --git a/runtime/vm/thread_pool_test.cc b/runtime/vm/thread_pool_test.cc
index 23d614094d5e488787553bd67374b48711f7bb91..2d5a7e2961170f26e6f518c021fcbb386bcfe80e 100644
--- a/runtime/vm/thread_pool_test.cc
+++ b/runtime/vm/thread_pool_test.cc
@@ -9,7 +9,7 @@
 
 namespace dart {
 
-DECLARE_FLAG(int, worker_timeout_millis);
+DECLARE_FLAG(int, worker_timeout_micros);
 
 
 UNIT_TEST_CASE(ThreadPool_Create) {
@@ -159,8 +159,8 @@ UNIT_TEST_CASE(ThreadPool_WorkerShutdown) {
 
 UNIT_TEST_CASE(ThreadPool_WorkerTimeout) {
   // Adjust the worker timeout so that we timeout quickly.
-  int saved_timeout = FLAG_worker_timeout_millis;
-  FLAG_worker_timeout_millis = 1;
+  int saved_timeout = FLAG_worker_timeout_micros;
+  FLAG_worker_timeout_micros = 1000;
 
   ThreadPool thread_pool;
   EXPECT_EQ(0U, thread_pool.workers_started());
@@ -190,7 +190,7 @@ UNIT_TEST_CASE(ThreadPool_WorkerTimeout) {
     waited += 1;
   }
   EXPECT_EQ(1U, thread_pool.workers_stopped());
-  FLAG_worker_timeout_millis = saved_timeout;
+  FLAG_worker_timeout_micros = saved_timeout;
 }