Created a DownloadManager interface, for use in unit tests..

content/shell/shell_browser_context.cc

Index: content/shell/shell_browser_context.cc
diff --git a/content/shell/shell_browser_context.cc b/content/shell/shell_browser_context.cc
index 23960fbcb08017cab58119c294d0a6b201dc4100..0f75d89138a01b152b1261bb9401d608722fa694 100644
--- a/content/shell/shell_browser_context.cc
+++ b/content/shell/shell_browser_context.cc
@@ -11,7 +11,7 @@
 #include "content/browser/appcache/chrome_appcache_service.h"
 #include "content/browser/chrome_blob_storage_context.h"
 #include "content/browser/download/download_id_factory.h"
-#include "content/browser/download/download_manager.h"
+#include "content/browser/download/download_manager_impl.h"
 #include "content/browser/download/download_status_updater.h"
 #include "content/browser/file_system/browser_file_system_helper.h"
 #include "content/browser/geolocation/geolocation_permission_context.h"
@@ -127,9 +127,9 @@ DownloadManager* ShellBrowserContext::GetDownloadManager()  {
     download_status_updater_.reset(new DownloadStatusUpdater());
 
     download_manager_delegate_ = new ShellDownloadManagerDelegate();
-    download_manager_ = new DownloadManager(download_manager_delegate_,
-                                            download_id_factory_,
-                                            download_status_updater_.get());
+    download_manager_ = new DownloadManagerImpl(download_manager_delegate_,
+                                                download_id_factory_,
+                                                download_status_updater_.get());
     download_manager_delegate_->SetDownloadManager(download_manager_.get());
     download_manager_->Init(this);
   }