GitLab

Relanding since the CL wasn't the cause of test flakiness.

Original change's description:
> [Extensions] Introduce common client initialization
>
> In order to extract apps-specific code from extensions, we'll need to
> be able to add extra information to the extensions client from outside
> extensions code. This will require having a centralized initialization
> for the extensions client.
>
> Introduce a single method, EnsureExtensionsClientInitialized(), which
> constructs and sets the ChromeExtensionsClient. Remove the old way of
> constructing the client, ChromeExtensionsClient::GetInstance(). Update
> all callers to construct the client via the new method.
>
> While we're at it, update the client to use base::NoDestructor instead
> of LazyInstance.
>
> Bug: 873937
> Change-Id: I47147cd5f33ea44079c0a319626c57f3c5813af8
> Reviewed-on: https://chromium-review.googlesource.com/1175459
> Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
> Reviewed-by: Lei Zhang <thestig@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#583217}

TBR=thestig@chromium.org,rdevlin.cronin@chromium.org

Bug: 873937
Change-Id: Idf8fdc2ca89e3f0e481fbfd310c5ebc6067311f1
Reviewed-on: https://chromium-review.googlesource.com/1181162Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584461}