Forward declare some fxjs/ classes.

And remove unnecessary fxjs/ #includes in header files.

Change-Id: If89dd2717b6ba93dbb7493e8ca80d78deaa64b06
Reviewed-on: https://pdfium-review.googlesource.com/c/44270
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/fxjs/xfa/cjx_subform.cpp b/fxjs/xfa/cjx_subform.cpp
index c08d2e6..2fd0957 100644
--- a/fxjs/xfa/cjx_subform.cpp
+++ b/fxjs/xfa/cjx_subform.cpp
@@ -8,6 +8,7 @@
 
 #include <vector>
 
+#include "fxjs/cfx_v8.h"
 #include "fxjs/cfxjse_value.h"
 #include "fxjs/js_resources.h"
 #include "xfa/fxfa/cxfa_eventparam.h"