Refactor SkGLContext to be actually extendable

include/gpu/gl/SkANGLEGLContext.h

Index: include/gpu/gl/SkANGLEGLContext.h
diff --git a/include/gpu/gl/SkANGLEGLContext.h b/include/gpu/gl/SkANGLEGLContext.h
index 18cdbdb213a71ae6a7c579d9e66e8654ce1dd1a9..797e997972a7982ce449d53e73cb854708debfca 100644
--- a/include/gpu/gl/SkANGLEGLContext.h
+++ b/include/gpu/gl/SkANGLEGLContext.h
@@ -17,19 +17,26 @@
 
 class SkANGLEGLContext : public SkGLContext {
 public:
-    SkANGLEGLContext();
-
-    virtual ~SkANGLEGLContext();
-
+    virtual ~SkANGLEGLContext() 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 SkANGLEGLContext* Create(GrGLStandard forcedGpuAPI) {
+        if (kGL_GrGLStandard == forcedGpuAPI) {
+            return NULL;
+        }
+        SkANGLEGLContext* ctx = SkNEW(SkANGLEGLContext);
+        if (!ctx->isValid()) {
+            SkDELETE(ctx);
+            return NULL;
+        }
+        return ctx;
+    }
 
 private:
+    SkANGLEGLContext();
+    void destroyGLContext();
+
     EGLContext fContext;
     EGLDisplay fDisplay;
     EGLSurface fSurface;