Rename App to Env where needed.

More instances of CPDFSDK_Environment being called m_pApp in the code. Renamed
to m_pEnv to be clear that it's the environment not an app object.

Review-Url: https://codereview.chromium.org/2354413002
diff --git a/fpdfsdk/javascript/PublicMethods.cpp b/fpdfsdk/javascript/PublicMethods.cpp
index 435c383..65af6a1 100644
--- a/fpdfsdk/javascript/PublicMethods.cpp
+++ b/fpdfsdk/javascript/PublicMethods.cpp
@@ -80,9 +80,9 @@
 }
 
 void AlertIfPossible(CJS_Context* pContext, const FX_WCHAR* swMsg) {
-  CPDFSDK_Environment* pApp = pContext->GetReaderApp();
-  if (pApp)
-    pApp->JS_appAlert(swMsg, nullptr, 0, 3);
+  CPDFSDK_Environment* pEnv = pContext->GetReaderEnv();
+  if (pEnv)
+    pEnv->JS_appAlert(swMsg, nullptr, 0, 3);
 }
 
 }  // namespace