Downgrade raw_span<> to span<> in StringViewTemplate.

Trade some small amount of temporal safety for performance gains,
say, when making repeated Substr() calls in a loop.

Change-Id: I637cd8c5f5eb13ecd272369d8988edb851293915
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/117211
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Thomas Sepez <tsepez@google.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>

core/fxcrt/string_view_template.h

diff --git a/core/fxcrt/string_view_template.h b/core/fxcrt/string_view_template.h
index 0730ae7..feb4fcd 100644
--- a/core/fxcrt/string_view_template.h
+++ b/core/fxcrt/string_view_template.h
@@ -16,7 +16,6 @@
 
 #include "core/fxcrt/fx_memcpy_wrappers.h"
 #include "core/fxcrt/fx_system.h"
-#include "core/fxcrt/raw_span.h"
 #include "core/fxcrt/span.h"
 
 namespace fxcrt {
@@ -267,7 +266,10 @@
   }
 
  protected:
-  pdfium::raw_span<const UnsignedType> m_Span;
+  // This is not a raw_span<> because StringViewTemplates must be passed by
+  // value without introducing BackupRefPtr churn. Also, repeated re-assignment
+  // of substrings of a StringViewTemplate to itself must avoid the same issue.
+  pdfium::span<const UnsignedType> m_Span;
 
  private:
   void* operator new(size_t) throw() { return nullptr; }