Send IPP printer-resolution to CUPS on Chrome OS

The CUPS resolution field, represented as quality in the print preview
dialog, was previously ignored on Chrome OS machines in favor of the
advanced print quality setting. This commit changes that - when
possible, Chrome will pass the selected resolution on to CUPS so that
print filters can print at the correct resolution.

Bug: 1001398
Test: ./printing_unittests
Change-Id: I3666591da07022d3dd02b2fadaaaf4835ad453b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2390967Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Pranav Batra <batrapranav@chromium.org>
Auto-Submit: Pranav Batra <batrapranav@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804492}

printing/printing_context_chromeos.cc

@@ -140,6 +140,13 @@ std::vector<ScopedCupsOption> SettingsToCupsOptions(
     options.push_back(ConstructOption(kIppPinEncryption, kPinEncryptionNone));
   }
 
+  if (settings.dpi_horizontal() > 0 && settings.dpi_vertical() > 0) {
+    std::string dpi = base::NumberToString(settings.dpi_horizontal());
+    if (settings.dpi_horizontal() != settings.dpi_vertical())
+      dpi += "x" + base::NumberToString(settings.dpi_vertical());
+    options.push_back(ConstructOption(kIppResolution, dpi + "dpi"));
+  }
+
   if (base::FeatureList::IsEnabled(
           printing::features::kAdvancedPpdAttributes)) {
     size_t regular_attr_count = options.size();