Internally measure isolate uptime with monotonic time.

runtime/vm/pages.cc

Index: runtime/vm/pages.cc
diff --git a/runtime/vm/pages.cc b/runtime/vm/pages.cc
index 4fb4efbc83e487722bfd602fd5329815239bbc77..8ddb372f8245b0477e86dcd9ac34602dbdebb861 100644
--- a/runtime/vm/pages.cc
+++ b/runtime/vm/pages.cc
@@ -704,7 +704,7 @@ void PageSpace::PrintToJSONObject(JSONObject* object) const {
   space.AddProperty64("external", ExternalInWords() * kWordSize);
   space.AddProperty("time", MicrosecondsToSeconds(gc_time_micros()));
   if (collections() > 0) {
-    int64_t run_time = OS::GetCurrentTimeMicros() - isolate->start_time();
+    int64_t run_time = isolate->UptimeMicros();
     run_time = Utils::Maximum(run_time, static_cast<int64_t>(0));
     double run_time_millis = MicrosecondsToMilliseconds(run_time);
     double avg_time_between_collections =
@@ -1144,8 +1144,8 @@ bool PageSpaceController::NeedsGarbageCollection(SpaceUsage after) const {
   // kInitialTimeoutSeconds, gradually lower the capacity limit.
   static const double kInitialTimeoutSeconds = 1.00;
   if (history_.IsEmpty()) {
-    double seconds_since_init = MicrosecondsToSeconds(
-        OS::GetCurrentTimeMicros() - heap_->isolate()->start_time());
+    double seconds_since_init =
+        MicrosecondsToSeconds(heap_->isolate()->UptimeMicros());
     if (seconds_since_init > kInitialTimeoutSeconds) {
       multiplier *= seconds_since_init / kInitialTimeoutSeconds;
     }