Rename CPDFXFA_Document to CPDFXFA_Context

The CPDFXFA_Document class isn't a document, it contains documents. Renamed
to make the purpose a bit clearer.

Review-Url: https://codereview.chromium.org/2469813004
diff --git a/fpdfsdk/cpdfsdk_interform.cpp b/fpdfsdk/cpdfsdk_interform.cpp
index 81836f7..a8024fa 100644
--- a/fpdfsdk/cpdfsdk_interform.cpp
+++ b/fpdfsdk/cpdfsdk_interform.cpp
@@ -37,7 +37,7 @@
 
 #ifdef PDF_ENABLE_XFA
 #include "fpdfsdk/cpdfsdk_xfawidget.h"
-#include "fpdfsdk/fpdfxfa/cpdfxfa_document.h"
+#include "fpdfsdk/fpdfxfa/cpdfxfa_context.h"
 #include "fpdfsdk/fpdfxfa/cxfa_fwladaptertimermgr.h"
 #include "xfa/fxfa/cxfa_eventparam.h"
 #include "xfa/fxfa/xfa_ffdocview.h"