Adds option to enable CSS backgrounds for printing.

printing/printing_context.cc

Index: printing/printing_context.cc
diff --git a/printing/printing_context.cc b/printing/printing_context.cc
index 09a93ff18e0e44219453984651871457389f8bd5..a29b31a6947dab4b9db58d7d62364ce6f5adc8c4 100644
--- a/printing/printing_context.cc
+++ b/printing/printing_context.cc
@@ -78,6 +78,12 @@ PrintingContext::Result PrintingContext::UpdatePrintSettings(
 
   PrintingContext::Result result = UpdatePrinterSettings(job_settings, ranges);
   PrintSettingsInitializer::InitHeaderFooterStrings(job_settings, &settings_);
+
+  if (!job_settings.GetBoolean(kSettingShouldPrintBackgrounds,
+                               &settings_.should_print_backgrounds)) {
+    NOTREACHED();
+  }
+
   return result;
 }