Mark pdfium::span<> as TRIVIAL_ABI and GSL_POINTER

Unlike base::span<>, pdfium::span<> is based on UnownedPtr<>, and
can benefit from being passed in registers despite having an
actual destructor.

Change-Id: I08bf1b342d246cb5f62eee5ff9722e5fb0471032
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/99231
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>

third_party/base/span.h

diff --git a/third_party/base/span.h b/third_party/base/span.h
index ec9f990..1c9c934 100644
--- a/third_party/base/span.h
+++ b/third_party/base/span.h
@@ -15,6 +15,7 @@
 
 #include "core/fxcrt/unowned_ptr.h"
 #include "third_party/base/check.h"
+#include "third_party/base/compiler_specific.h"
 
 namespace pdfium {
 
@@ -176,7 +177,7 @@
 
 // [span], class template span
 template <typename T>
-class span {
+class TRIVIAL_ABI GSL_POINTER span {
  public:
   using value_type = typename std::remove_cv<T>::type;
   using pointer = T*;