Split DownloadItem into an ABC, an Impl, and a Mock.

chrome/browser/download/download_item_unittest.cc

Index: chrome/browser/download/download_item_unittest.cc
diff --git a/chrome/browser/download/download_item_unittest.cc b/chrome/browser/download/download_item_unittest.cc
index 45f634b60c80fa984c69551bb27a21a4cc33f552..99a50125854039b4bdab2a7a6e4d5041d684b159 100644
--- a/chrome/browser/download/download_item_unittest.cc
+++ b/chrome/browser/download/download_item_unittest.cc
@@ -10,6 +10,7 @@
 #include "content/browser/download/download_item.h"
 #include "content/browser/download/download_status_updater.h"
 #include "content/browser/download/interrupt_reasons.h"
+#include "content/browser/download/mock_download_item.h"
 #include "content/browser/download/mock_download_manager.h"
 #include "content/browser/download/mock_download_manager_delegate.h"
 #include "content/test/test_browser_thread.h"
@@ -251,3 +252,7 @@ TEST_F(DownloadItemTest, NotificationAfterTogglePause) {
   item->TogglePause();
   ASSERT_TRUE(observer.CheckUpdated());
 }
+
+TEST(MockDownloadItem, Compiles) {
+  MockDownloadItem mock_item;
+}