Fix build with use_system_libpng=true.

This is a follow-up to https://pdfium-review.googlesource.com/90050,
to convert uses of FXCODEC_RESUNIT_* that were not part of the default
build.

Change-Id: I6a5e4e2eecf0e25f4a38a7f105581d102c460d9a
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/90634
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/core/fxcodec/png/png_decoder.cpp b/core/fxcodec/png/png_decoder.cpp
index ffdd7e7..4046169 100644
--- a/core/fxcodec/png/png_decoder.cpp
+++ b/core/fxcodec/png/png_decoder.cpp
@@ -58,10 +58,10 @@
     png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y, &unit_type);
     switch (unit_type) {
       case PNG_RESOLUTION_METER:
-        pAttribute->m_wDPIUnit = FXCODEC_RESUNIT_METER;
+        pAttribute->m_wDPIUnit = CFX_DIBAttribute::kResUnitMeter;
         break;
       default:
-        pAttribute->m_wDPIUnit = FXCODEC_RESUNIT_NONE;
+        pAttribute->m_wDPIUnit = CFX_DIBAttribute::kResUnitNone;
     }
 #endif
 #if defined(PNG_iCCP_SUPPORTED)