Fix GN build after PNG support backport
Fix GN build after PNG support backport

Follow-on from https://codereview.chromium.org/950113002.
This would block a pdfium roll until corrected.

TBR=jam@chromium.org

Review URL: https://codereview.chromium.org/948233002

was4444/pdfium,nathanstitt/pdfium,andoma/pdfium,csharpengineer/pdfium-code,was4444/pdfium,andoma/pdfium,duanhjlt/pdfium,DrAlexx/pdfium,ttyangf/PDFium-,ttyangf/PDFium-,azunite/libpdfium,bradh/pdfium,azunite/pdfium_ch,azunite/libpdfium,rouault/pdfium,bradh/pdfium,azunite/pdf_clone,DrAlexx/pdfium,bradh/pdfium,chenjian-rits/HelloWorld,DrAlexx/pdfium,azunite/pdf_clone,wwxxyx/pdfium0,DrAlexx/pdfium,klokantech/pdfium,ttyangf/PDFium-,azunite/pdf_clone,rouault/pdfium,duanhjlt/pdfium,duanhjlt/pdfium,rouault/pdfium,nathanstitt/pdfium,klokantech/pdfium,chenjian-rits/HelloWorld,csharpengineer/pdfium-code,pwaller/pdfium,wwxxyx/pdfium0,azunite/libpdfium,chenjian-rits/HelloWorld,was4444/pdfium,andoma/pdfium,azunite/pdfium_ch,nathanstitt/pdfium,azunite/pdfium_ch,klokantech/pdfium,pwaller/pdfium,wwxxyx/pdfium0,pwaller/pdfium,andoma/pdfium,azunite/libpdfium,duanhjlt/pdfium,was4444/pdfium,csharpengineer/pdfium-code