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_util.cpp b/fxjs/cjs_util.cpp
index 7efb840..e868fcc 100644
--- a/fxjs/cjs_util.cpp
+++ b/fxjs/cjs_util.cpp
@@ -112,7 +112,7 @@
   {
     size_t offset = 0;
     while (true) {
-      absl::optional<size_t> offset_end =
+      std::optional<size_t> offset_end =
           unsafe_fmt_string.Find(L"%", offset + 1);
       if (!offset_end.has_value()) {
         unsafe_conversion_specifiers.push_back(