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

runtime/vm/scope_timer.h

Index: runtime/vm/scope_timer.h
diff --git a/runtime/vm/scope_timer.h b/runtime/vm/scope_timer.h
index 1266f292793cdced4a671e59dbaf57b8dbed3c97..c8952254c20a7c0ff0e47c63166c036ea3f118ff 100644
--- a/runtime/vm/scope_timer.h
+++ b/runtime/vm/scope_timer.h
@@ -17,11 +17,11 @@ class ScopeTimer : public ValueObject {
     if (!enabled_) {
       return;
     }
-    start_ = OS::GetCurrentTimeMicros();
+    start_ = OS::GetCurrentMonotonicMicros();
   }
 
   int64_t GetElapsed() const {
-    int64_t end = OS::GetCurrentTimeMicros();
+    int64_t end = OS::GetCurrentMonotonicMicros();
     ASSERT(end >= start_);
     return end - start_;
   }