Implement SkGLContext swapBuffers with fence syncs

include/gpu/gl/angle/SkANGLEGLContext.h

Index: include/gpu/gl/angle/SkANGLEGLContext.h
diff --git a/include/gpu/gl/angle/SkANGLEGLContext.h b/include/gpu/gl/angle/SkANGLEGLContext.h
index f54f29ecbac4dbe03a0860049880fbb4869b85c0..8850cd0bb5518f02dd92d5869cfb80ee0496d3ba 100644
--- a/include/gpu/gl/angle/SkANGLEGLContext.h
+++ b/include/gpu/gl/angle/SkANGLEGLContext.h
@@ -15,8 +15,6 @@
 class SkANGLEGLContext : public SkGLContext {
 public:
     ~SkANGLEGLContext() override;
-    void makeCurrent() const override;
-    void swapBuffers() const override;
 
     static SkANGLEGLContext* Create(GrGLStandard forcedGpuAPI) {
         if (kGL_GrGLStandard == forcedGpuAPI) {
@@ -37,6 +35,10 @@ private:
     SkANGLEGLContext();
     void destroyGLContext();
 
+    void onPlatformMakeCurrent() const override;
+    void onPlatformSwapBuffers() const override;
+    GrGLFuncPtr onPlatformGetProcAddress(const char* name) const override;
+
     void* fContext;
     void* fDisplay;
     void* fSurface;