Manually load CONDITION_VARIABLE methods on Windows, checking for failure (XP).

src/core/SkTaskGroup.cpp

 #include "SkTaskGroup.h"
 
 #include "SkCondVar.h"
 #include "SkRunnable.h"
 #include "SkTDArray.h"
 #include "SkThread.h"
 #include "SkThreadUtils.h"
 
 #if defined(SK_BUILD_FOR_WIN32)
     static inline int num_cores() {
@@ skipping 147 matching lines @@
 
     static ThreadPool* gGlobal;
     friend struct SkTaskGroup::Enabler;
 };
 ThreadPool* ThreadPool::gGlobal = NULL;
 
 }  // namespace
 
 SkTaskGroup::Enabler::Enabler(int threads) {
     SkASSERT(ThreadPool::gGlobal == NULL);
-    if (threads != 0) {
+    if (threads != 0 && SkCondVar::Supported()) {
         ThreadPool::gGlobal = SkNEW_ARGS(ThreadPool, (threads));
     }
 }
 
 SkTaskGroup::Enabler::~Enabler() {
     SkDELETE(ThreadPool::gGlobal);
 }
 
 SkTaskGroup::SkTaskGroup() : fPending(0) {}
 
 void SkTaskGroup::wait()                            { ThreadPool::Wait(&fPending); }
 void SkTaskGroup::add(SkRunnable* task)             { ThreadPool::Add(task, &fPending); }
 void SkTaskGroup::add(void (*fn)(void*), void* arg) { ThreadPool::Add(fn, arg, &fPending); }
 void SkTaskGroup::batch (void (*fn)(void*), void* args, int N, size_t stride) {
     ThreadPool::Batch(fn, args, N, stride, &fPending);
 }