[Autofill] Add the second form signature to proto

This CL doesn't add second form signature calculation. What should be included in the second form signature is to be researched more. See discussion in https://chromium-review.googlesource.com/c/chromium/src/+/1221248


Bug: 552420
Change-Id: I79c9d27995233943fb704bec5268f97941475c21
Reviewed-on: https://chromium-review.googlesource.com/1241155
Reviewed-by: Roger McFarlane <rogerm@chromium.org>
Commit-Queue: Maxim Kolosovskiy <kolos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594100}

components/autofill/core/browser/proto/server.proto

@@ -179,11 +179,16 @@ message AutofillRandomizedFieldMetadata {
 
 // This message contains information about the field types in a single form.
 // It is sent by the toolbar to contribute to the field type statistics.
-// Next available id: 34
+// Next available id: 35
 message AutofillUploadContents {
   required string client_version = 1;
   required fixed64 form_signature = 2;
 
+  // The secondary form signature is calculated based on field types instead of
+  // names and is used if the primary one is unstable, i.e. the field names
+  // change on every page load.
+  optional fixed64 secondary_form_signature = 34;
+
   // True if the autofill feature was used to fill this form, false otherwise.
   required bool autofill_used = 3;