Merge to XFA: Fix (nearly all) IWYU in fpdfskd/include/javascript/ headers.

Original Review URL: https://codereview.chromium.org/1060133002
TBR=thestig@chromium.org

Review URL: https://codereview.chromium.org/1065103002
diff --git a/fpdfsdk/include/javascript/JS_EventHandler.h b/fpdfsdk/include/javascript/JS_EventHandler.h
index d16933a..a6736d3 100644
--- a/fpdfsdk/include/javascript/JS_EventHandler.h
+++ b/fpdfsdk/include/javascript/JS_EventHandler.h
@@ -7,7 +7,14 @@
 #ifndef _JS_EVENTHANDLER_H_
 #define _JS_EVENTHANDLER_H_
 
+#include "../../../core/include/fxcrt/fx_string.h"
+#include "../../../core/include/fxcrt/fx_system.h"
+
 class CJS_Context;
+class CPDFSDK_Annot;
+class CPDFSDK_Document;
+class CPDF_Bookmark;
+class CPDF_FormField;
 class Field;
 
 enum JS_EVENT_T