Make idle tasks optional in the default platform.

test/unittests/libplatform/default-platform-unittest.cc

Index: test/unittests/libplatform/default-platform-unittest.cc
diff --git a/test/unittests/libplatform/default-platform-unittest.cc b/test/unittests/libplatform/default-platform-unittest.cc
index 473f8d39b827d5a594ddaf9f58c476b3b615416d..73816596cf42fd8dce337657b4323a28dabde7c2 100644
--- a/test/unittests/libplatform/default-platform-unittest.cc
+++ b/test/unittests/libplatform/default-platform-unittest.cc
@@ -27,7 +27,8 @@ struct MockIdleTask : public IdleTask {
 
 class DefaultPlatformWithMockTime : public DefaultPlatform {
  public:
-  DefaultPlatformWithMockTime() : time_(0) {}
+  DefaultPlatformWithMockTime()
+      : DefaultPlatform(IdleTaskSupport::kEnabled), time_(0) {}
   double MonotonicallyIncreasingTime() override { return time_; }
   void IncreaseTime(double seconds) { time_ += seconds; }