Convert some CPDF_ExpIntFunc getters to return spans.

Use spans to help enforce bound checking for GetBeginValues() and
GetEndValues(). Also mark these methods as Skia-only.

Change-Id: I7589cee0db26c7073f72e1d733eb29ae7d9fd837
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/97930
Reviewed-by: Nigi <nigi@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

core/fpdfapi/page/cpdf_expintfunc.h

diff --git a/core/fpdfapi/page/cpdf_expintfunc.h b/core/fpdfapi/page/cpdf_expintfunc.h
index c31ad8b..d829456 100644
--- a/core/fpdfapi/page/cpdf_expintfunc.h
+++ b/core/fpdfapi/page/cpdf_expintfunc.h
@@ -12,6 +12,10 @@
 
 #include "core/fpdfapi/page/cpdf_function.h"
 
+#if defined(_SKIA_SUPPORT_) || defined(_SKIA_SUPPORT_PATHS_)
+#include "third_party/base/span.h"
+#endif
+
 class CPDF_ExpIntFunc final : public CPDF_Function {
  public:
   CPDF_ExpIntFunc();
@@ -25,8 +29,11 @@
 
   uint32_t GetOrigOutputs() const { return m_nOrigOutputs; }
   float GetExponent() const { return m_Exponent; }
-  const std::vector<float>& GetBeginValues() const { return m_BeginValues; }
-  const std::vector<float>& GetEndValues() const { return m_EndValues; }
+
+#if defined(_SKIA_SUPPORT_) || defined(_SKIA_SUPPORT_PATHS_)
+  pdfium::span<const float> GetBeginValues() const { return m_BeginValues; }
+  pdfium::span<const float> GetEndValues() const { return m_EndValues; }
+#endif
 
  private:
   uint32_t m_nOrigOutputs = 0;

core/fxge/skia/fx_skia_device.cpp

diff --git a/core/fxge/skia/fx_skia_device.cpp b/core/fxge/skia/fx_skia_device.cpp
index 80f3266..d153bc4 100644
--- a/core/fxge/skia/fx_skia_device.cpp
+++ b/core/fxge/skia/fx_skia_device.cpp
@@ -419,8 +419,8 @@
   if (pFunc->GetOrigOutputs() != 3)
     return false;
 
-  auto begin_values = pFunc->GetBeginValues().begin();
-  auto end_values = pFunc->GetEndValues().begin();
+  auto begin_values = pFunc->GetBeginValues();
+  auto end_values = pFunc->GetEndValues();
   if (is_encode_reversed)
     std::swap(begin_values, end_values);