Merge to XFA: Do some IWYU cleanups.

R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1176333002.

(cherry picked from commit 0ef0de55657db8a83372ad8eb22d84c5893afc4c)

Review URL: https://codereview.chromium.org/1195943005.
diff --git a/testing/fx_string_testhelpers.h b/testing/fx_string_testhelpers.h
index 7d06688..13b4afb 100644
--- a/testing/fx_string_testhelpers.h
+++ b/testing/fx_string_testhelpers.h
@@ -7,7 +7,7 @@
 
 #include <ostream>
 
-#include "../core/include/fxcrt/fx_basic.h"
+#include "../core/include/fxcrt/fx_string.h"
 
 // Output stream operator so GTEST macros work with FX strings.
 std::ostream& operator<<(std::ostream& out, const CFX_ByteStringC& str);