[Autofill Assistant] Precondition proto number.

Before this patch, the client and server protos differed in the
precondition field numbering. This resulted in fields being empty.

This patch fixes that.

After this patch, preconditions have properly populated fields after
parsing.

R=rouslan@chromium.org

Bug: 806868
Change-Id: Iec4dcfe6ad9668394a9b16b11719f185dd259e47
Reviewed-on: https://chromium-review.googlesource.com/1249208Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Commit-Queue: Mathias Carlen <mcarlen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594808}

components/autofill_assistant/browser/service.proto

@@ -46,7 +46,7 @@ message SupportedScriptProto {
 
     // Precondition contains a set of conditions that must hold for a script to
     // be executed. No precondition means that a script can run in any case.
-    optional ScriptPreconditionProto precondition = 2;
+    optional ScriptPreconditionProto precondition = 3;
 
     // Display priority of the script. Lowest number has highest priority, which
     // means a script with priority 0 should be displayed before a script with