Fixing bug when sending last_document argument to cupsStartDestDocument

The SendDocument function in cups_printer which calls
cupsStartDestDocument in CUPS had the logic backwards for passing the
"last_document" argument. This was causing CUPS to behave strangely and
mishandle printing multiple copies of documents for printers which did
not support the copies IPP attribute.

Bug: chromium:897275
Test: Tested manually using elm device and virtual USB printer
Change-Id: I313965bfe4c281adf071ef8c14a022df4e0c2c12
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1695693
Auto-Submit: David Valleau <valleau@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Commit-Queue: David Valleau <valleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676090}

printing/backend/cups_printer.cc

@@ -178,7 +178,7 @@ bool CupsPrinter::StartDocument(int job_id,
   http_status_t start_doc_status =
       cupsStartDestDocument(cups_http_, destination_.get(), dest_info_.get(),
                             job_id, document_name.c_str(), CUPS_FORMAT_PDF,
-                            options.size(), data, last_document ? 0 : 1);
+                            options.size(), data, last_document ? 1 : 0);
 
   cupsSetUser(nullptr);  // reset to default username ("anonymous")
   return start_doc_status == HTTP_CONTINUE;