Modernize and trim down SkOnce.

src/core/SkTaskGroup.cpp

Index: src/core/SkTaskGroup.cpp
diff --git a/src/core/SkTaskGroup.cpp b/src/core/SkTaskGroup.cpp
index b696555d6faef35394199a1c658ab39990f31649..2ae1b5997128d514d97ce7693a60816061065941 100644
--- a/src/core/SkTaskGroup.cpp
+++ b/src/core/SkTaskGroup.cpp
@@ -26,11 +26,11 @@
     }
 #endif
 
-// We cache sk_num_cores() so we only query the OS once.
-SK_DECLARE_STATIC_ONCE(g_query_num_cores_once);
 int sk_num_cores() {
+    // We cache sk_num_cores() so we only query the OS once.
     static int num_cores = 0;
-    SkOnce(&g_query_num_cores_once, query_num_cores, &num_cores);
+    static SkOnce once;

[bungeman-skia, 2016/04/18 15:19:00]

It is not entirely clear to me how using member initializers solves the issue of
initializing this value.

[mtklein, 2016/04/18 15:27:44]

It makes the default constructor for SkOnce constexpr.  The SkOnce comes
pre-initialized.

+    once(query_num_cores, &num_cores);
     SkASSERT(num_cores > 0);
     return num_cores;
 }