Perform full GC in background idle notification.

BUG=chromium:515873
LOG=NO

Review URL: https://codereview.chromium.org/1263373002

Cr-Commit-Position: refs/heads/master@{#29996}

src/heap/memory-reducer.cc

@@ -95,6 +95,12 @@ void MemoryReducer::NotifyBackgroundIdleNotification(const Event& event) {
   if (old_action == kWait && state_.action == kWait &&
       old_started_gcs + 1 == state_.started_gcs) {
     DCHECK(heap()->incremental_marking()->IsStopped());
+    // TODO(ulan): Replace it with incremental marking GC once
+    // chromium:490559 is fixed.
+    if (event.time_ms > state_.last_gc_time_ms + kLongDelayMs) {
+      heap()->CollectAllGarbage(Heap::kReduceMemoryFootprintMask,
+                                "memory reducer background GC");
+    } else {
       DCHECK(FLAG_incremental_marking);
       heap()->StartIdleIncrementalMarking();
       if (FLAG_trace_gc_verbose) {
@@ -104,6 +110,7 @@ void MemoryReducer::NotifyBackgroundIdleNotification(const Event& event) {
                      state_.started_gcs);
       }
     }
+  }
 }