Store attachment crc in AttachmentStore

sync/internal_api/attachments/proto/attachment_store.proto

Index: sync/internal_api/attachments/proto/attachment_store.proto
diff --git a/sync/internal_api/attachments/proto/attachment_store.proto b/sync/internal_api/attachments/proto/attachment_store.proto
index 86f90715593464118bbf87c27637731512116fc2..7172f46d830069bedd6ea7fe82b4b57a3f5e11a3 100644
--- a/sync/internal_api/attachments/proto/attachment_store.proto
+++ b/sync/internal_api/attachments/proto/attachment_store.proto
@@ -24,4 +24,6 @@ message StoreMetadata {
 message RecordMetadata {
   // Size of attachment data. Useful for attachment store space management.
   optional int64 attachment_size = 1;
+  // Crc of attachment data.

[maniscalco, 2014/11/11 00:44:54]

Crc -> crc32c

[pavely, 2014/11/11 22:27:15]

Done.

+  optional fixed32 crc32c = 2;

[maniscalco, 2014/11/11 00:44:54]

Why use fixed32?  I would have expected uint32.

[pavely, 2014/11/11 22:27:15]

Protobuf language guide says that fixed32 "Always four bytes. More efficient
than uint32 if values are often greater than 2^28."

[maniscalco, 2014/11/11 23:05:32]

Wow, so many integral types!  Looks like uint32 is more common in chrome protos,
but I'm OK with fixed32 as well.  Up to you.

 }