[heap] Do not require kRingBufferMaxSize samples for estimating compaction speed

There is no need to require kRingbufferMaxSize samples for estimating the
compaction speed, as the number is already quite stable with a single sample
(which may include timings from compacting multiple pages).

R=hpayer@chromium.org
BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1440423002

Cr-Commit-Position: refs/heads/master@{#31993}

src/heap/gc-tracer.cc

@@ -731,7 +731,7 @@ intptr_t GCTracer::ScavengeSpeedInBytesPerMillisecond(
 
 
 intptr_t GCTracer::CompactionSpeedInBytesPerMillisecond() const {
-  if (compaction_events_.size() < kRingBufferMaxSize) return 0;
+  if (compaction_events_.size() == 0) return 0;
   intptr_t bytes = 0;
   double durations = 0.0;
   CompactionEventBuffer::const_iterator iter = compaction_events_.begin();