[Background Fetch] Update persistent storage tasks w/metadata.

Update the database manager task wrappers to use the metadata proto directly.

The following files were moved and slightly updated to make naming consistent throughout:
  * create_registration_task.(cc|h) -> create_metadata_task.(cc|h)
  * get_registration_task.(cc|h) -> get_metadata_task.(cc|h)
The metadata to registration conversion logic was removed from get_registration_task.cc.

Also added tests.

Bug: 826257
Change-Id: I7a8cedf76df3b935ebeddf6536ba5cda639420db
Reviewed-on: https://chromium-review.googlesource.com/993054
Commit-Queue: Rayan Kanso <[email protected]>
Reviewed-by: Dan Elphick <[email protected]>
Reviewed-by: Camille Lamy <[email protected]>
Cr-Commit-Position: refs/heads/master@{#549645}
diff --git a/content/browser/BUILD.gn b/content/browser/BUILD.gn
index e35a5b8..d8867f0 100644
--- a/content/browser/BUILD.gn
+++ b/content/browser/BUILD.gn
@@ -426,8 +426,8 @@
     "background_fetch/background_fetch_service_impl.h",
     "background_fetch/storage/cleanup_task.cc",
     "background_fetch/storage/cleanup_task.h",
-    "background_fetch/storage/create_registration_task.cc",
-    "background_fetch/storage/create_registration_task.h",
+    "background_fetch/storage/create_metadata_task.cc",
+    "background_fetch/storage/create_metadata_task.h",
     "background_fetch/storage/database_helpers.cc",
     "background_fetch/storage/database_helpers.h",
     "background_fetch/storage/database_task.cc",
@@ -436,8 +436,8 @@
     "background_fetch/storage/delete_registration_task.h",
     "background_fetch/storage/get_developer_ids_task.cc",
     "background_fetch/storage/get_developer_ids_task.h",
-    "background_fetch/storage/get_registration_task.cc",
-    "background_fetch/storage/get_registration_task.h",
+    "background_fetch/storage/get_metadata_task.cc",
+    "background_fetch/storage/get_metadata_task.h",
     "background_fetch/storage/mark_registration_for_deletion_task.cc",
     "background_fetch/storage/mark_registration_for_deletion_task.h",
     "background_fetch/storage/update_registration_ui_task.cc",