Add back default SerialExecutor defaulted to getGlobalCPUExecutor.

Summary: Use the global immutable executor by default for SerialExecutor.

Reviewed By: yfeldblum

Differential Revision: D28925750

fbshipit-source-id: 91f75cfb3a4880098d933fe1f148d5c3b2e896e7

folly/executors/SerialExecutor.h

@@ -57,7 +57,8 @@ class SerialExecutor : public SequencedExecutor {
   SerialExecutor(SerialExecutor&&) = delete;
   SerialExecutor& operator=(SerialExecutor&&) = delete;
 
-  static KeepAlive<SerialExecutor> create(KeepAlive<Executor> parent);
+  static KeepAlive<SerialExecutor> create(
+      KeepAlive<Executor> parent = getGlobalCPUExecutor());
 
   class Deleter {
    public:

folly/executors/test/SerialExecutorTest.cpp

@@ -179,8 +179,7 @@ TEST(SerialExecutor, RecursiveAddInline) {
 }
 
 TEST(SerialExecutor, ExecutionThrows) {
-  auto executor = SerialExecutor::create(folly::getKeepAliveToken(
-      folly::getUnsafeMutableGlobalCPUExecutor().get()));
+  auto executor = SerialExecutor::create();
 
   // an empty Func will throw std::bad_function_call when invoked,
   // but SerialExecutor should catch that exception