Build pdfium_test executable under GN.
Build pdfium_test executable under GN.

This is the first side of a two-sided patch;
the dependency on //third_party/pdfium/samples will be included from //pdf/BUILD.gn in the chrome repo.

BUG=https://code.google.com/p/pdfium/issues/detail?id=77
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/720193004

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