[Downloads] Introduce GUIDs for downloads.

content/browser/download/download_item_impl.h

Index: content/browser/download/download_item_impl.h
diff --git a/content/browser/download/download_item_impl.h b/content/browser/download/download_item_impl.h
index 18ba4495dca85e21b6688d232db26cb617acf8e2..3e4ce85867a1b1a5002f5318f509c03ceb427f6c 100644
--- a/content/browser/download/download_item_impl.h
+++ b/content/browser/download/download_item_impl.h
@@ -52,6 +52,7 @@ class CONTENT_EXPORT DownloadItemImpl
   // Constructing from persistent store:
   // |bound_net_log| is constructed externally for our use.
   DownloadItemImpl(DownloadItemImplDelegate* delegate,
+                   const std::string& guid,
                    uint32_t id,
                    const base::FilePath& current_path,
                    const base::FilePath& target_path,
@@ -105,6 +106,7 @@ class CONTENT_EXPORT DownloadItemImpl
   void OpenDownload() override;
   void ShowDownloadInShell() override;
   uint32_t GetId() const override;
+  const std::string& GetGuid() const override;
   DownloadState GetState() const override;
   DownloadInterruptReason GetLastReason() const override;
   bool IsPaused() const override;
@@ -480,6 +482,8 @@ class CONTENT_EXPORT DownloadItemImpl
   // download system.
   scoped_ptr<DownloadRequestHandleInterface> request_handle_;
 
+  std::string guid_;
+
   uint32_t download_id_ = kInvalidId;
 
   // Display name for the download. If this is empty, then the display name is