Refactor SkGLContext to be actually extendable

src/gpu/gl/SkNullGLContext.cpp

Index: src/gpu/gl/SkNullGLContext.cpp
diff --git a/src/gpu/gl/SkNullGLContext.cpp b/src/gpu/gl/SkNullGLContext.cpp
index 576ee526f19811843265cf17ece837271b5c3940..68807e7c574d5b5a819d204345ae2c7e3639d6e5 100644
--- a/src/gpu/gl/SkNullGLContext.cpp
+++ b/src/gpu/gl/SkNullGLContext.cpp
@@ -8,9 +8,11 @@
 
 #include "gl/SkNullGLContext.h"
 
-const GrGLInterface* SkNullGLContext::createGLContext(GrGLStandard forcedGpuAPI) {
-    if (kGLES_GrGLStandard == forcedGpuAPI) {
-        return NULL;
-    }
-    return GrGLCreateNullInterface();
-};
+SkNullGLContext::SkNullGLContext() {
+    fGL.reset(GrGLCreateNullInterface());
+}
+
+SkNullGLContext::~SkNullGLContext() {
+    fGL.reset(NULL);
+}
+