Fix compile on mac: format string mismatch error.

See http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_compile_dbg/builds/9542/steps/compile%20%28with%20patch%29/logs/stdio

TBR=jun_fang@foxitsoftware.com,

Review URL: https://codereview.chromium.org/506053003

samples/pdfium_test.cc

diff --git a/samples/pdfium_test.cc b/samples/pdfium_test.cc
index 7436e9d..0f0c875 100644
--- a/samples/pdfium_test.cc
+++ b/samples/pdfium_test.cc
@@ -359,8 +359,8 @@
   FPDF_CloseDocument(doc);
   FPDFAvail_Destroy(pdf_avail);
 
-  printf("Loaded, parsed and rendered %d pages.\n", rendered_pages);
-  printf("Skipped %d bad pages.\n", bad_pages);
+  printf("Loaded, parsed and rendered %zu pages.\n", rendered_pages);
+  printf("Skipped %zu bad pages.\n", bad_pages);
 }
 
 int main(int argc, const char* argv[]) {