Increase the delay of memory reducer to make it less likely to start GC

when the application is not idle.

BUG=

Review URL: https://codereview.chromium.org/1369333005

Cr-Commit-Position: refs/heads/master@{#31100}

src/heap/memory-reducer.cc

@@ -13,7 +13,7 @@
 namespace v8 {
 namespace internal {
 
-const int MemoryReducer::kLongDelayMs = 5000;
+const int MemoryReducer::kLongDelayMs = 20000;
 const int MemoryReducer::kShortDelayMs = 500;
 const int MemoryReducer::kWatchdogDelayMs = 100000;
 const int MemoryReducer::kMaxNumberOfGCs = 3;