GitLab

Move ManifestHandler registration out of the body of
ExtensionsClient::Initialize() and into the ExtensionsAPIProviders by
introducing ExtensionsAPIProvider::RegisterManifestHandlers().

Have each provider register their associated ManifestHandlers, and have
a common ExtensionsClient initialization step call into them in order
to trigger the registration.

Bug: 873937

Change-Id: I8c400efd9d91968a8dd28b249483ded31bbc79e0
Reviewed-on: https://chromium-review.googlesource.com/1177190Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585386}