GitLab

This is a reland of 65478d12 with
Aura-specific tests moved to a subclass and #ifdef.

Original change's description:
> AppShell: Support reloading
>
> Implement reloading in ExtensionLoader. Uses keep-alives while apps are
> reloading. A future CL will observe these keep-alives to keep app_shell open
> during reload.
>
> Bug: 762642
> Change-Id: Ia20b81378d1aeab4ace119d9d2a8b44c2461fdcf
> Reviewed-on: https://chromium-review.googlesource.com/912694
> Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
> Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#540079}

Bug: 762642
Change-Id: I6c516c4736ca2327965e034d10717341e23303dc
Reviewed-on: https://chromium-review.googlesource.com/950342
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542195}