add a download slices table into history download db

components/history/core/browser/download_database.h

Index: components/history/core/browser/download_database.h
diff --git a/components/history/core/browser/download_database.h b/components/history/core/browser/download_database.h
index 6388195eccf2b1b0f314fe86c21487ce10a6fd21..6f67d4aed6c1b673b3181127f6fd73fd03f31686 100644
--- a/components/history/core/browser/download_database.h
+++ b/components/history/core/browser/download_database.h
@@ -14,6 +14,7 @@
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/threading/platform_thread.h"
+#include "components/history/core/browser/download_job_info.h"
 #include "components/history/core/browser/download_types.h"
 
 namespace sql {
@@ -27,6 +28,9 @@ struct DownloadRow;
 // Maintains a table of downloads.
 class DownloadDatabase {
  public:
+  // Check if Parallel downloading feature is enabled.
+  static bool IsParallelDownloadingEnabled();
+
   // Must call InitDownloadTable before using any other functions.
   DownloadDatabase(DownloadInterruptReason download_interrupt_reason_none,
                    DownloadInterruptReason download_interrupt_reason_crash);
@@ -113,6 +117,16 @@ class DownloadDatabase {
 
   void RemoveDownloadURLs(uint32_t id);
 
+  bool EnsureDownloadJobsTableExists();
+
+  // Updates the state of a download job. Returns true on success, or false
+  // otherwise. If the job doesn't exist, a new job will be created. If
+  // |is_new_download| is true, should always insert a new job into the db.
+  bool UpdateDownloadJob(const DownloadJobInfo& info, bool is_new_download);
+
+  // Delete all the download jobs associated with one DownloadRow.
+  void RemoveDownloadJobs(uint32_t id);
+
   bool owning_thread_set_;
   base::PlatformThreadId owning_thread_;