Switch from absl::optional to std::optional

Since absl::optional is using std::optional underneath, just switch to
using std::optional directly.

Change-Id: Ide39da5ac9c7551a710e653a60e15f022046df7b
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/116290
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Thomas Sepez <tsepez@google.com>
diff --git a/fxjs/cjs_runtime.cpp b/fxjs/cjs_runtime.cpp
index f7d76da..aba2346 100644
--- a/fxjs/cjs_runtime.cpp
+++ b/fxjs/cjs_runtime.cpp
@@ -166,7 +166,7 @@
   return m_pFormFillEnv.Get();
 }
 
-absl::optional<IJS_Runtime::JS_Error> CJS_Runtime::ExecuteScript(
+std::optional<IJS_Runtime::JS_Error> CJS_Runtime::ExecuteScript(
     const WideString& script) {
   return Execute(script);
 }