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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}