Allow more scavenges in idle notification by increasing the new space limit distance.

BUG=chromium:468554
LOG=n

Review URL: https://codereview.chromium.org/1034403002

Cr-Commit-Position: refs/heads/master@{#27517}

src/heap/gc-idle-time-handler.cc

@@ -131,8 +131,12 @@ bool GCIdleTimeHandler::ShouldDoScavenge(
         static_cast<size_t>(new_space_size * kConservativeTimeRatio);
   } else {
     // We have to trigger scavenge before we reach the end of new space.
-    new_space_allocation_limit -=
-        new_space_allocation_throughput_in_bytes_per_ms * kMaxScheduledIdleTime;
+    size_t adjust_limit = new_space_allocation_throughput_in_bytes_per_ms *
+                          kTimeUntilNextIdleEvent;
+    if (adjust_limit > new_space_allocation_limit)
+      new_space_allocation_limit = 0;
+    else
+      new_space_allocation_limit -= adjust_limit;
   }
 
   if (scavenge_speed_in_bytes_per_ms == 0) {

src/heap/gc-idle-time-handler.h

@@ -122,6 +122,10 @@ class GCIdleTimeHandler {
   // 16 ms when there is currently no rendering going on.
   static const size_t kMaxScheduledIdleTime = 50;
 
+  // We conservatively assume that in the next kTimeUntilNextIdleEvent ms
+  // no idle notification happens.
+  static const size_t kTimeUntilNextIdleEvent = 100;
+
   // If we haven't recorded any scavenger events yet, we use a conservative
   // lower bound for the scavenger speed.
   static const size_t kInitialConservativeScavengeSpeed = 100 * KB;

test/unittests/heap/gc-idle-time-handler-unittest.cc

@@ -136,7 +136,7 @@ TEST_F(GCIdleTimeHandlerTest, DoScavengeUnknownScavengeSpeed) {
   GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
   heap_state.used_new_space_size = kNewSpaceCapacity;
   heap_state.scavenge_speed_in_bytes_per_ms = 0;
-  int idle_time_in_ms = 16;
+  int idle_time_in_ms = 8;
   EXPECT_FALSE(GCIdleTimeHandler::ShouldDoScavenge(
       idle_time_in_ms, heap_state.new_space_capacity,
       heap_state.used_new_space_size, heap_state.scavenge_speed_in_bytes_per_ms,