Revert "Remove skstd::optional entirely."

This reverts commit 0596094b81.

Reason for revert: Flutter somehow still references skstd::optional

flutter_engine in google3 still has old code:
http://screen/BHDrjqwzchdFVfQ

Original change's description:
> Remove skstd::optional entirely.
>
> Change-Id: Id9862712ea3e769797abd654922879ce6bc4487c
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/504976
> Reviewed-by: Ben Wagner <bungeman@google.com>
> Reviewed-by: Brian Osman <brianosman@google.com>
> Commit-Queue: John Stiles <johnstiles@google.com>

Change-Id: Idea391399c2e11b83d5a130023adb340d40cadcb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/505396
Auto-Submit: John Stiles <johnstiles@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Heather Miller <hcm@google.com>
Commit-Queue: Heather Miller <hcm@google.com>

gn/core.gni

@@ -502,6 +502,7 @@ skia_core_sources = [
   "$_include/private/SkTFitsIn.h",
   "$_include/private/SkTHash.h",
   "$_include/private/SkTLogic.h",
+  "$_include/private/SkTOptional.h",
   "$_include/private/SkTemplates.h",
   "$_include/private/SkThreadAnnotations.h",
   "$_include/private/SkThreadID.h",

include/core/SkCanvas.h

@@ -28,10 +28,10 @@
 #include "include/core/SkTypes.h"
 #include "include/private/SkDeque.h"
 #include "include/private/SkMacros.h"
+#include "include/private/SkTOptional.h"
 
 #include <cstring>
 #include <memory>
-#include <optional>
 #include <vector>
 
 #ifndef SK_SUPPORT_LEGACY_GETTOTALMATRIX

include/core/SkImage.h

@@ -15,11 +15,11 @@
 #include "include/core/SkScalar.h"
 #include "include/core/SkShader.h"
 #include "include/core/SkTileMode.h"
+#include "include/private/SkTOptional.h"
 #if SK_SUPPORT_GPU
 #include "include/gpu/GrTypes.h"
 #endif
 #include <functional>  // std::function
-#include <optional>
 
 #if defined(SK_BUILD_FOR_ANDROID) && __ANDROID_API__ >= 26
 #include <android/hardware_buffer.h>

include/core/SkImageGenerator.h

@@ -13,8 +13,7 @@
 #include "include/core/SkImage.h"
 #include "include/core/SkImageInfo.h"
 #include "include/core/SkYUVAPixmaps.h"
-
-#include <optional>
+#include "include/private/SkTOptional.h"
 
 class GrRecordingContext;
 class GrSurfaceProxyView;

include/core/SkPaint.h

@@ -11,10 +11,9 @@
 #include "include/core/SkBlendMode.h"
 #include "include/core/SkColor.h"
 #include "include/core/SkRefCnt.h"
+#include "include/private/SkTOptional.h"
 #include "include/private/SkTo.h"
 
-#include <optional>
-
 class SkBlender;
 class SkColorFilter;
 class SkColorSpace;

include/effects/SkRuntimeEffect.h

@@ -18,9 +18,9 @@
 #include "include/core/SkString.h"
 #include "include/private/SkOnce.h"
 #include "include/private/SkSLSampleUsage.h"
+#include "include/private/SkTOptional.h"
 
 #include <string>
-#include <optional>
 #include <vector>
 
 #ifdef SK_ENABLE_SKSL

include/private/SkTOptional.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2021 Google LLC.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkTOptional_DEFINED
+#define SkTOptional_DEFINED
+
+#include <optional>
+
+namespace skstd {
+
+template <typename T> using optional = std::optional<T>;
+using nullopt_t = std::nullopt_t;
+inline constexpr nullopt_t nullopt = std::nullopt;
+
+} // namespace skstd
+
+#endif