Disallow absl::optional usage via include_rules

Since PDFium switched over to std::optional, update include_rules to ban
absl/types/optional.h. While this diverges from Chromium's rules, the
divergence is temporary, as Chromium is doing the same migration in
https://crbug.com/40288126 and will make this change when they are done.

Change-Id: Iacb339ff52aacc91c50117c1b3f49193b87eaa3c
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/116310
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Thomas Sepez <tsepez@google.com>

DEPS

diff --git a/DEPS b/DEPS
index 8874cce..efb8201 100644
--- a/DEPS
+++ b/DEPS
@@ -532,6 +532,7 @@
   '-third_party/abseil-cpp/absl/synchronization',
   '-third_party/abseil-cpp/absl/time',
   '-third_party/abseil-cpp/absl/types/any.h',
+  '-third_party/abseil-cpp/absl/types/optional.h',
   '-third_party/abseil-cpp/absl/types/span.h',
 ]