Use iosfwd in more headers.

Forward declaring shaves compilation costs down 1%.

Change-Id: Iea0c1d73db25327ebd4de8e895f912181774a670
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/79991
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/testing/fx_string_testhelpers.cpp b/testing/fx_string_testhelpers.cpp
index 23a8e98..51983de 100644
--- a/testing/fx_string_testhelpers.cpp
+++ b/testing/fx_string_testhelpers.cpp
@@ -6,6 +6,7 @@
 
 #include <iomanip>
 #include <ios>
+#include <ostream>
 
 #include "core/fxcrt/cfx_datetime.h"
 #include "core/fxcrt/fx_string.h"