Implement SkGLContext swapBuffers with fence syncs

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 ca71ddeb0e1677bd3e138516e295cad8c8bf4b46..1f634382641ec02dd336304f754a80c1ac78b5e2 100644
--- a/include/gpu/gl/SkNullGLContext.h
+++ b/include/gpu/gl/SkNullGLContext.h
@@ -13,8 +13,6 @@
 class SK_API SkNullGLContext : public SkGLContext {
 public:
     ~SkNullGLContext() override;
-    void makeCurrent() const override;
-    void swapBuffers() const override {};
 
     static SkNullGLContext* Create(GrGLStandard);
 
@@ -23,6 +21,10 @@ public:
 private:
     SkNullGLContext();
 
+    void onPlatformMakeCurrent() const override;
+    void onPlatformSwapBuffers() const override {}
+    GrGLFuncPtr onPlatformGetProcAddress(const char*) const override { return NULL; }
+
     ContextState* fState;
 };