[Autofill Assistant] Proto field numbering fix.

Make the element reference proto consistent with what the backend
expects.

R=rouslan@chromium.org, szermatt@chromium.org

Bug: 806868
Change-Id: I2125add6b931d2ccce5fe2dc1986709a087237ee
Reviewed-on: https://chromium-review.googlesource.com/1240297Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Commit-Queue: Mathias Carlen <mcarlen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593642}

components/autofill_assistant/browser/service.proto

@@ -213,7 +213,7 @@ message ElementReferenceProto {
   // document.
   // APIs are free to reject element references that do not refer to unique
   // elements (i.e. resolve to more than one element on the page).
-  repeated string selectors = 1;
+  repeated string selectors = 2;
 }
 
 // Contain all arguments to perform a click.