Refactor SkGLContext to be actually extendable

include/gpu/gl/SkNullGLContext.h

Index: include/gpu/gl/SkNullGLContext.h
diff --git a/include/gpu/gl/SkNullGLContext.h b/include/gpu/gl/SkNullGLContext.h
index 16621fc69675f848fa3176b64507d7e49bdabd74..35e94d2ab93d9e1338a8a8036dba2f15fd1d3b09 100644
--- a/include/gpu/gl/SkNullGLContext.h
+++ b/include/gpu/gl/SkNullGLContext.h
@@ -11,18 +11,25 @@
 #include "SkGLContext.h"
 
 class SK_API SkNullGLContext : public SkGLContext {
-
 public:
-    SkNullGLContext() {};
-
+    virtual ~SkNullGLContext() SK_OVERRIDE;
     virtual void makeCurrent() const SK_OVERRIDE {};
-
     virtual void swapBuffers() const SK_OVERRIDE {};
 
-protected:
-    virtual const GrGLInterface* createGLContext(GrGLStandard forcedGpuAPI) SK_OVERRIDE;
-
-    virtual void destroyGLContext() SK_OVERRIDE {};
+    static SkNullGLContext* Create(GrGLStandard forcedGpuAPI) {
+        if (kGLES_GrGLStandard == forcedGpuAPI) {
+            return NULL;
+        }
+        SkNullGLContext* ctx = SkNEW(SkNullGLContext);
+        if (!ctx->isValid()) {
+            SkDELETE(ctx);
+            return NULL;
+        }
+        return ctx;
+    }
+
+private:
+    SkNullGLContext();
 };
 
 #endif