Temporarily revert to non-incremental GC in background tabs to see

if that is responsible for memory regression in UMA.

Related CLs: crrev.com/1420363004, crrev.com/1423453003

TBR=hpayer@chromium.org
BUG=chromium:552305
LOG=NO

Review URL: https://codereview.chromium.org/1420283021

Cr-Commit-Position: refs/heads/master@{#31853}

src/heap/memory-reducer.cc

@@ -73,7 +73,14 @@ void MemoryReducer::NotifyTimer(const Event& event) {
       PrintIsolate(heap()->isolate(), "Memory reducer: started GC #%d\n",
                    state_.started_gcs);
     }
-    heap()->StartIdleIncrementalMarking();
+    if (heap()->ShouldOptimizeForMemoryUsage()) {
+      // TODO(ulan): Remove this once crbug.com/552305 is fixed.
+      // Do full GC if memory usage has higher priority than latency.
+      heap()->CollectAllGarbage(Heap::kReduceMemoryFootprintMask,
+                                "memory reducer");
+    } else {
+      heap()->StartIdleIncrementalMarking();
+    }
   } else if (state_.action == kWait) {
     if (!heap()->incremental_marking()->IsStopped() &&
         heap()->ShouldOptimizeForMemoryUsage()) {