Add printing.dll to the component build.

printing/backend/print_backend.h

Index: printing/backend/print_backend.h
===================================================================
--- printing/backend/print_backend.h	(revision 99019)
+++ printing/backend/print_backend.h	(working copy)
@@ -11,6 +11,7 @@
 #include <vector>
 
 #include "base/memory/ref_counted.h"
+#include "printing/printing_export.h"
 
 namespace base {
 class DictionaryValue;
@@ -19,7 +20,7 @@
 // This is the interface for platform-specific code for a print backend
 namespace printing {
 
-struct PrinterBasicInfo {
+struct PRINTING_EXPORT PrinterBasicInfo {
   PrinterBasicInfo();
   ~PrinterBasicInfo();
 
@@ -32,7 +33,7 @@
 
 typedef std::vector<PrinterBasicInfo> PrinterList;
 
-struct PrinterCapsAndDefaults {
+struct PRINTING_EXPORT PrinterCapsAndDefaults {
   PrinterCapsAndDefaults();
   ~PrinterCapsAndDefaults();
 
@@ -49,7 +50,8 @@
 // print system specific. For example, CUPS is available on both Linux and Mac,
 // but not available on ChromeOS, etc. This design allows us to add more
 // functionality on some platforms, while reusing core (CUPS) functions.
-class PrintBackend : public base::RefCountedThreadSafe<PrintBackend> {
+class PRINTING_EXPORT PrintBackend
+    : public base::RefCountedThreadSafe<PrintBackend> {
  public:
   virtual ~PrintBackend();