[autofill] Rich autofill queries behind a flag.

This CL adds rich metadata to Autofill queries for the purpose of
testing autofill field-type prediction models on the autofill server.
This functionality is limited to non-release channels (i.e., Canary
and Dev, but *NOT* Beta or Stable).

When enabled, Chrome will augment the autofill queries with the
plaintext values of the following form and field metadata:

  For the form:
  * id attribute
  * name attribute

  For each field
  * id attribute
  * name attribute
  * control type (i.e., text, tel, select, etc)
  * label
  * aria label
  * aria description
  * class attribute
  * placeholder attribute

This data will be used by the autofill server to predict the types
of each of the form's fields, for testing and evaluation purposes.

TBR: [email protected]
Bug: 898510
Change-Id: I3f7bc9f550e638137b9e2b34c1142932f791e3ef
Reviewed-on: https://chromium-review.googlesource.com/c/1334090
Commit-Queue: Roger McFarlane <[email protected]>
Reviewed-by: Fabio Tirelo <[email protected]>
Reviewed-by: Sebastien Seguin-Gagnon <[email protected]>
Reviewed-by: Moe Ahmadi <[email protected]>
Cr-Commit-Position: refs/heads/master@{#610484}
diff --git a/chrome/browser/flag_descriptions.cc b/chrome/browser/flag_descriptions.cc
index cce95e9..979e40a 100644
--- a/chrome/browser/flag_descriptions.cc
+++ b/chrome/browser/flag_descriptions.cc
@@ -139,6 +139,12 @@
 const char kAutofillRestrictUnownedFieldsToFormlessCheckoutDescription[] =
     "Restrict extraction of formless forms to checkout flows";
 
+const char kAutofillRichMetadataQueriesName[] =
+    "Autofill - Rich metadata queries (Canary/Dev only)";
+const char kAutofillRichMetadataQueriesDescription[] =
+    "Transmit rich form/field metadata when querying the autofill server. "
+    "This feature only works on the Canary and Dev channels.";
+
 const char kAutoplayPolicyName[] = "Autoplay policy";
 const char kAutoplayPolicyDescription[] =
     "Policy used when deciding if audio or video is allowed to autoplay.";