[Sync] Use a proto to generate AutofillSyncStorageKey's storage keys.

components/autofill/core/browser/proto/BUILD.gn

Index: components/autofill/core/browser/proto/BUILD.gn
diff --git a/components/autofill/core/browser/proto/BUILD.gn b/components/autofill/core/browser/proto/BUILD.gn
index ad52e78646b3a2edf44ea16e992e3d8187eb2dad..2a8069f637866621e10e2133bf3e9c9fce0c95cb 100644
--- a/components/autofill/core/browser/proto/BUILD.gn
+++ b/components/autofill/core/browser/proto/BUILD.gn
@@ -6,6 +6,7 @@ import("//third_party/protobuf/proto_library.gni")
 
 proto_library("proto") {
   sources = [
+    "autofill_sync.proto",

[Mathieu, 2017/01/06 01:51:29]

why not "sync.proto"? We are already in the autofill namespace/directory

[skym, 2017/01/06 18:29:03]

You're right, it is redundant. I actually tried to do this when I started. But
the include guard that is generated is solely based on the relative path passed
into protoc, and I couldn't figure out an easy way to tweak that. So we conflict
with the existing sync.proto file. Would be nice if it took into account the
proto "package" field when constructing the guard string, oh well.

     "server.proto",
   ]
 }