Merge to XFA: Fix all relative includes to core.

Do some IWYU to fix build errors due to files that have no #includes but
just happened to work previously because the #includes were in the right
order.

R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1407423004 .

(cherry picked from commit 9e7cd8ffb81c564e1102f6c6459ec0942a0b11ea)

Review URL: https://codereview.chromium.org/1425163004 .
diff --git a/fpdfsdk/include/fsdk_mgr.h b/fpdfsdk/include/fsdk_mgr.h
index 76d7adf..709a137 100644
--- a/fpdfsdk/include/fsdk_mgr.h
+++ b/fpdfsdk/include/fsdk_mgr.h
@@ -9,7 +9,7 @@
 
 #include <map>
 
-#include "../../core/include/fpdftext/fpdf_text.h"
+#include "core/include/fpdftext/fpdf_text.h"
 #include "fpdfxfa/fpdfxfa_doc.h"
 #include "fpdfxfa/fpdfxfa_page.h"
 #include "fsdk_actionhandler.h"