[Autofill Assistant] Default backend url.

Before this patch, autofill assistant was managed via a server-side
experiment and parameter for the url. This patch default enables the
feature since external callers are responsible for starting autofill
assistant via parameters. This simplifies the reasoning about the
responsibilities. This means the backend url is also added in this
patch and there is a single source of truth on the c++ side for it.

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

Bug: 806868
Change-Id: I686f087e7c8625b962e6b6c80a2acba238f183f8
Reviewed-on: https://chromium-review.googlesource.com/c/1369792
Commit-Queue: Mathias Carlen <mcarlen@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Stephane Zermatten <szermatt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616276}
5 files changed