GitLab

api_test_utils::CreateExtension() variants are no longer really useful
with the new-and-improved ExtensionBuilder class. Remove them to reduce
code and have one fewer ways of constructing test extension objects.

While we're at it, const-ify the creation sites.

TBR=atwilson@chromium.org
(just updating background_mode_manager_unittest to use a builder)

Bug: 889925
Change-Id: I99087834e6b4c30a43c0badb0245295e0e1de525
Reviewed-on: https://chromium-review.googlesource.com/1252766
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595488}