Return GrGLFunction from gles_bind().

GrGLFunction is a std::function today, but we'd like to slim that down.
This will let us change its underlying type seamlessly.

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I9f9c3d4b56e6d6fb6db82c8f47ddf74fdca6476d
Reviewed-on: https://chromium-review.googlesource.com/580129Reviewed-by: Brian Salomon <bsalomon@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488394}

gpu/skia_bindings/gl_bindings_skia_cmd_buffer.cc

@@ -11,7 +11,7 @@ using gpu::gles2::GLES2Interface;
 
 namespace {
 template <typename R, typename... Args>
-std::function<R(Args...)> gles_bind(R (GLES2Interface::*func)(Args...),
+GrGLFunction<R (*)(Args...)> gles_bind(R (GLES2Interface::*func)(Args...),
                                        GLES2Interface* gles2Interface) {
   return [func, gles2Interface](Args... args) {
     return (gles2Interface->*func)(args...);