Double the API call rate threshold for the memory reducer to kick in

Moving Blink from hidden values to private symbols almost doubled the
rate of API calls, as the private symbols API increases the call-depth
scope while the hidden values one didn't.

I manually checked that this fixes the regression on the long running
gmail benchmark but doesn't add new memory reducer GCs on the infinite
scrolling benchmark

BUG=chromium:561325
R=ulan@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1472193003

Cr-Commit-Position: refs/heads/master@{#32279}

src/heap/memory-reducer.cc

@@ -24,7 +24,7 @@ MemoryReducer::TimerTask::TimerTask(MemoryReducer* memory_reducer)
 
 
 void MemoryReducer::TimerTask::RunInternal() {
-  const double kJsCallsPerMsThreshold = 0.25;
+  const double kJsCallsPerMsThreshold = 0.5;
   Heap* heap = memory_reducer_->heap();
   Event event;
   double time_ms = heap->MonotonicallyIncreasingTimeInMs();