Add IdleTask API to v8::Platform.

src/libplatform/default-platform.cc

Index: src/libplatform/default-platform.cc
diff --git a/src/libplatform/default-platform.cc b/src/libplatform/default-platform.cc
index b41c5852a82119aaa074e6b74b7b5c47e957df97..ddceab54572d12f3a4f5ce6db9ced6675e8f6ed4 100644
--- a/src/libplatform/default-platform.cc
+++ b/src/libplatform/default-platform.cc
@@ -155,6 +155,15 @@ void DefaultPlatform::CallDelayedOnForegroundThread(Isolate* isolate,
 }
 
 
+void DefaultPlatform::CallIdleOnForegroundThread(Isolate* isolate,
+                                                 IdleTask* task) {
+  UNREACHABLE();
+}
+
+
+bool DefaultPlatform::IdleTasksEnabled(Isolate* isolate) { return false; }
+
+
 double DefaultPlatform::MonotonicallyIncreasingTime() {
   return base::TimeTicks::HighResolutionNow().ToInternalValue() /
          static_cast<double>(base::Time::kMicrosecondsPerSecond);