Restore a really single-threaded mode to DM.

src/utils/SkTaskGroup.cpp

 #include "SkTaskGroup.h"
 
 #include "SkCondVar.h"
 #include "SkTDArray.h"
 #include "SkThread.h"
 #include "SkThreadUtils.h"
 
 #if defined(SK_BUILD_FOR_WIN32)
     static inline int num_cores() {
         SYSTEM_INFO sysinfo;
@@ skipping 49 matching lines @@
     private:
         SkCondVar* fC;
     };
 
     struct Work {
         SkRunnable* task;  // A task to ->run(),
         int32_t* pending;  // then sk_atomic_dec(pending) afterwards.
     };
 
     explicit ThreadPool(int threads) : fDraining(false) {
-        if (threads == 0) {
+        if (threads == -1) {
             threads = num_cores();
         }
         for (int i = 0; i < threads; i++) {
             fThreads.push(SkNEW_ARGS(SkThread, (&ThreadPool::Loop, this)));
             fThreads.top()->start();
         }
     }
 
     ~ThreadPool() {
         SkASSERT(fWork.isEmpty());  // All SkTaskGroups should be destroyed by now.
@@ skipping 45 matching lines @@
 
     static ThreadPool* gGlobal;
     friend struct SkTaskGroup::Enabler;
 };
 ThreadPool* ThreadPool::gGlobal = NULL;
 
 }  // namespace
 
 SkTaskGroup::Enabler::Enabler(int threads) {
     SkASSERT(ThreadPool::gGlobal == NULL);
-    ThreadPool::gGlobal = SkNEW_ARGS(ThreadPool, (threads));
+    if (threads != 0) {
+        ThreadPool::gGlobal = SkNEW_ARGS(ThreadPool, (threads));
+    }
 }
 
 SkTaskGroup::Enabler::~Enabler() {
-    SkASSERT(ThreadPool::gGlobal != NULL);
     SkDELETE(ThreadPool::gGlobal);
 }
 
 SkTaskGroup::SkTaskGroup() : fPending(0) {}
 
 void SkTaskGroup::add(SkRunnable* task) { ThreadPool::Add(task, &fPending); }
 void SkTaskGroup::wait()                { ThreadPool::Wait(&fPending); }