[Autofill] Add |is_form_tag| to the server proto

Whether an uploaded form is a <form> tag or a list of unowned <input>s may be useful for server side analysis.
In particular, the list of unowned <input>s can contain several semantic forms (e.g. a search field,  sign-in form, sign-up form). The server side classifiers may apply special heuristics for the <form>less forms.

Bug: 552420, 910546
Change-Id: If4a5ec04eb1c0d87ec3a8cd399d84e3f5c31a692
Reviewed-on: https://chromium-review.googlesource.com/c/1394583
Commit-Queue: Maxim Kolosovskiy <kolos@chromium.org>
Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619772}

chrome/browser/autofill/autofill_server_browsertest.cc

@@ -216,6 +216,7 @@ IN_PROC_BROWSER_TEST_F(AutofillServerTest,
   upload.set_passwords_revealed(false);
   upload.set_submission_event(
       AutofillUploadContents_SubmissionIndicatorEvent_HTML_FORM_SUBMISSION);
+  upload.set_has_form_tag(true);
 
   test::FillUploadField(upload.add_field(), 2594484045U, "one", "text", nullptr,
                         2U);

components/autofill/core/browser/form_structure.cc

@@ -575,6 +575,7 @@ bool FormStructure::EncodeUploadRequest(
   upload->set_autofill_used(form_was_autofilled);
   upload->set_data_present(EncodeFieldTypes(available_field_types));
   upload->set_passwords_revealed(passwords_were_revealed_);
+  upload->set_has_form_tag(is_form_tag_);
   if (!page_language_.empty() && randomized_encoder_ != nullptr) {
     upload->set_language(page_language_);
   }

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

@@ -185,7 +185,7 @@ 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: 37
+// Next available id: 38
 message AutofillUploadContents {
   required string client_version = 1;
   required fixed64 form_signature = 2;
@@ -393,6 +393,9 @@ message AutofillUploadContents {
   // Titles of form's buttons.
   // TODO(850606): Deprecate once randomized metadata is launched.
   repeated ButtonTitle button_title = 36;
+
+  // Whether the fields are enclosed by a <form> tag or are unowned elements.
+  optional bool has_form_tag = 37;
 }
 
 // This proto contains information about the validity of each field in an