Draw forms properly in PDFiumEngine when rasterizing.

https://pdfium.googlesource.com/pdfium.git/+/75f84a56 changed the PDFium
rendering APIs to let callers better control how forms are drawn. The
path for low quality printing / print as image never got updated.

BUG=825985,pdfium:594

Change-Id: I36658f651da3911034648651c6118acbd4050712
Reviewed-on: https://chromium-review.googlesource.com/985133Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548344}

pdf/pdfium/pdfium_engine.cc

@@ -1550,7 +1550,13 @@ FPDF_DOCUMENT PDFiumEngine::CreateSinglePageRasterPdf(
   FPDF_RenderPageBitmap(bitmap, page_to_print->GetPrintPage(), page_rect.x(),
                         page_rect.y(), page_rect.width(), page_rect.height(),
                         print_settings.orientation,
-                        FPDF_ANNOT | FPDF_PRINTING | FPDF_NO_CATCH);
+                        FPDF_PRINTING | FPDF_NO_CATCH);
+
+  // Draw the forms.
+  FPDF_FFLDraw(form_, bitmap, page_to_print->GetPrintPage(), page_rect.x(),
+               page_rect.y(), page_rect.width(), page_rect.height(),
+               print_settings.orientation,
+               FPDF_ANNOT | FPDF_PRINTING | FPDF_NO_CATCH);
 
   unsigned char* bitmap_data =
       static_cast<unsigned char*>(FPDFBitmap_GetBuffer(bitmap));