Clean up singleton implementation

fpdfsdk/fpdfxfa/fpdfxfa_app.cpp

Index: fpdfsdk/fpdfxfa/fpdfxfa_app.cpp
diff --git a/fpdfsdk/fpdfxfa/fpdfxfa_app.cpp b/fpdfsdk/fpdfxfa/fpdfxfa_app.cpp
index 5fe114a64cca6bd6e33f050deaccfbdc8b02488f..e943c233be6193c7cfe9753b400632c7a668704e 100644
--- a/fpdfsdk/fpdfxfa/fpdfxfa_app.cpp
+++ b/fpdfsdk/fpdfxfa/fpdfxfa_app.cpp
@@ -13,18 +13,18 @@
 #include "xfa/fxfa/include/xfa_ffapp.h"
 #include "xfa/fxfa/include/xfa_fontmgr.h"
 
-CPDFXFA_App* CPDFXFA_App::g_pApp = nullptr;
+CPDFXFA_App* CPDFXFA_App::m_pApp = nullptr;
 
 CPDFXFA_App* CPDFXFA_App::GetInstance() {
-  if (!g_pApp) {
-    g_pApp = new CPDFXFA_App();
+  if (!m_pApp) {
+    m_pApp = new CPDFXFA_App();
   }
-  return g_pApp;
+  return m_pApp;
 }
 
 void CPDFXFA_App::ReleaseInstance() {
-  delete g_pApp;
-  g_pApp = nullptr;
+  delete m_pApp;
+  m_pApp = nullptr;
 }
 
 CPDFXFA_App::CPDFXFA_App()