Add export directives needed by Chromium WM project.

content/browser/renderer_host/image_transport_factory.h

Index: content/browser/renderer_host/image_transport_factory.h
diff --git a/content/browser/renderer_host/image_transport_factory.h b/content/browser/renderer_host/image_transport_factory.h
index 96587bd0bd9e9e490660b15825a72fe5e858143b..35f2dc6cd5294b7e0950aa861e01bce4c290b869 100644
--- a/content/browser/renderer_host/image_transport_factory.h
+++ b/content/browser/renderer_host/image_transport_factory.h
@@ -8,6 +8,7 @@
 #include <string>
 
 #include "base/memory/ref_counted.h"
+#include "content/common/content_export.h"
 #include "ui/gfx/native_widget_types.h"
 
 namespace gfx {
@@ -46,7 +47,7 @@ class ImageTransportFactoryObserver {
 // using that surface as a texture). The factory is a process-wide singleton.
 // As this is intimately linked to the type of 3D context we use (in-process or
 // command-buffer), implementations of this also implement ui::ContextFactory.
-class ImageTransportFactory {
+class CONTENT_EXPORT ImageTransportFactory {
  public:
   virtual ~ImageTransportFactory() {}