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

mutator utilization.

This makes GC heuristics more robust for small applications that trigger
scavenges but do not trigger full GC.

BUG=

Review URL: https://codereview.chromium.org/1280703002

Cr-Commit-Position: refs/heads/master@{#30060}

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