Use conservative estimate for GC speed instead of bailing out when computing

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index e37c9f6b11de8b333743e8f296486d3561721013..0de520db6b6cd4594df86b78e9a326e253c9d622 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -4685,7 +4685,9 @@ void Heap::MakeHeapIterable() {
 
 static double ComputeMutatorUtilization(double mutator_speed, double gc_speed) {
   const double kMinMutatorUtilization = 0.0;
-  if (mutator_speed == 0 || gc_speed == 0) return kMinMutatorUtilization;
+  const double kConservativeGcSpeedInBytesPerMillisecond = 200000;
+  if (mutator_speed == 0) return kMinMutatorUtilization;
+  if (gc_speed == 0) gc_speed = kConservativeGcSpeedInBytesPerMillisecond;
   // Derivation:
   // mutator_utilization = mutator_time / (mutator_time + gc_time)
   // mutator_time = 1 / mutator_speed