Rename m_pApp to m_pEnv

The m_pApp variable holds a CPDFSDK_Environment. This Cl renames the variable
to be m_pEnv so as not to confuse with the various App classes that exist.

Review-Url: https://codereview.chromium.org/2357293002
diff --git a/fpdfsdk/formfiller/cffl_checkbox.cpp b/fpdfsdk/formfiller/cffl_checkbox.cpp
index 9b9245d..ef1bf18 100644
--- a/fpdfsdk/formfiller/cffl_checkbox.cpp
+++ b/fpdfsdk/formfiller/cffl_checkbox.cpp
@@ -42,7 +42,7 @@
   switch (nChar) {
     case FWL_VKEY_Return:
     case FWL_VKEY_Space: {
-      CFFL_IFormFiller* pIFormFiller = m_pApp->GetIFormFiller();
+      CFFL_IFormFiller* pIFormFiller = m_pEnv->GetIFormFiller();
       ASSERT(pIFormFiller);
 
       CPDFSDK_PageView* pPageView = pAnnot->GetPageView();