Quota: Document the persistence of blink::mojom::StorageType.

The enum's values are persisted to disk. Make the values explicit, and
add a comment warning against deleting / renumbering values.

Change-Id: I10761ecd987e23f38302e201ad0345dccb05866e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1969246
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Jarryd Goodman <jarrydg@chromium.org>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#725417}

third_party/blink/public/mojom/quota/quota_types.mojom

@@ -4,12 +4,14 @@
 
 module blink.mojom;
 
+// Values are persisted to disk.
+// Do not renumber or delete existing values, to avoid data loss.
 enum StorageType {
-  kTemporary,
-  kPersistent,
-  kSyncable,
-  kQuotaNotManaged,
-  kUnknown,
+  kTemporary = 0,
+  kPersistent = 1,
+  kSyncable = 2,
+  kQuotaNotManaged = 3,
+  kUnknown = 4,
 };
 
 // These values need to match core/dom/exception_code.h.