Make code compile with clang_use_chrome_plugin (part I)

core/fpdfapi/fpdf_page/cpdf_shadingpattern.cpp

Index: core/fpdfapi/fpdf_page/cpdf_shadingpattern.cpp
diff --git a/core/fpdfapi/fpdf_page/cpdf_shadingpattern.cpp b/core/fpdfapi/fpdf_page/cpdf_shadingpattern.cpp
index 1636e17f4fabf552764b503b9e0934dbb3700d58..e2205f956d8608659760d6180cfad6ff771f38dd 100644
--- a/core/fpdfapi/fpdf_page/cpdf_shadingpattern.cpp
+++ b/core/fpdfapi/fpdf_page/cpdf_shadingpattern.cpp
@@ -50,6 +50,14 @@ CPDF_ShadingPattern::~CPDF_ShadingPattern() {
     m_pDocument->GetPageData()->ReleaseColorSpace(pCS->GetArray());
 }
 
+CPDF_TilingPattern* CPDF_ShadingPattern::AsTilingPattern() {
+  return nullptr;
+}
+
+CPDF_ShadingPattern* CPDF_ShadingPattern::AsShadingPattern() {
+  return this;
+}
+
 bool CPDF_ShadingPattern::Load() {
   if (m_ShadingType != kInvalidShading)
     return TRUE;