Rename PngModule to PngDecoder.

Change-Id: I11f79d77fedbe5c2f086b642309e3f4ecdf2c7ef
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/69918
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/core/fxcodec/BUILD.gn b/core/fxcodec/BUILD.gn
index 34023c6..b168112 100644
--- a/core/fxcodec/BUILD.gn
+++ b/core/fxcodec/BUILD.gn
@@ -113,8 +113,8 @@
     }
     if (pdf_enable_xfa_png) {
       sources += [
-        "png/pngmodule.cpp",
-        "png/pngmodule.h",
+        "png/png_decoder.cpp",
+        "png/png_decoder.h",
       ]
       deps += [ "../../third_party:png" ]
     }