Let SkTaskGroup work synchronously if no one created an SkTaskGroup::Enabler.

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;
         GetSystemInfo(&sysinfo);
         return sysinfo.dwNumberOfProcessors;
     }
 #else
     #include <unistd.h>
     static inline int num_cores() {
         return (int) sysconf(_SC_NPROCESSORS_ONLN);
     }
 #endif
 
 namespace {
 
 class ThreadPool : SkNoncopyable {
 public:
     static void Add(SkRunnable* task, int32_t* pending) {
-        SkASSERT(gGlobal);  // If this fails, see SkTaskGroup::Enabler.
+        if (!gGlobal) {  // If we have no threads, run synchronously.
+            return task->run();
+        }
         gGlobal->add(task, pending);
     }
 
     static void Wait(int32_t* pending) {
-        SkASSERT(gGlobal);  // If this fails, see SkTaskGroup::Enabler.
+        if (!gGlobal) {  // If we have no threads, the work must already be done.
+            SkASSERT(*pending == 0);
+            return;
+        }
         while (sk_acquire_load(pending) > 0) {  // Pairs with sk_atomic_dec here or in Loop.
             // Lend a hand until our SkTaskGroup of interest is done.
             Work work;
             {
                 AutoLock lock(&gGlobal->fReady);
                 if (gGlobal->fWork.isEmpty()) {
                     // Someone has picked up all the work (including ours).  How nice of them!
                     // (They may still be working on it, so we can't assert *pending == 0 here.)
                     continue;
                 }
@@ skipping 92 matching lines @@
 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); }