Implement SkGLContext swapBuffers with fence syncs

src/gpu/gl/mesa/SkMesaGLContext.h

Index: src/gpu/gl/mesa/SkMesaGLContext.h
diff --git a/src/gpu/gl/mesa/SkMesaGLContext.h b/src/gpu/gl/mesa/SkMesaGLContext.h
index bf0c7e9060480fc1f39d6f2486a16e48289a5a1c..fa3df7b7c7b16a69e1eb9b1313ba5eb0f9589335 100644
--- a/src/gpu/gl/mesa/SkMesaGLContext.h
+++ b/src/gpu/gl/mesa/SkMesaGLContext.h
@@ -18,8 +18,6 @@ private:
 
 public:
     ~SkMesaGLContext() override;
-    void makeCurrent() const override;
-    void swapBuffers() const override;
 
     static SkMesaGLContext* Create(GrGLStandard forcedGpuAPI) {
         if (kGLES_GrGLStandard == forcedGpuAPI) {
@@ -37,6 +35,10 @@ private:
     SkMesaGLContext();
     void destroyGLContext();
 
+    void onPlatformMakeCurrent() const override;
+    void onPlatformSwapBuffers() const override;
+    GrGLFuncPtr onPlatformGetProcAddress(const char*) const override;
+
     Context fContext;
     GrGLubyte *fImage;
 };