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The default implementation provides no subservices. Its purpose is to fulfill a dependency (in unit tests)
when other BrowserContextKeyedServices depend on an ExtensionSystem. Tests that actually need subservices
can override the necessary methods to provide them.

Also, this defines a MockExtensionSystemFactory template for ease of use with MockExtensionSystem subclasses.

BUG=348066

Review URL: https://codereview.chromium.org/460203002

Cr-Commit-Position: refs/heads/master@{#289235}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289235 0039d316-1c4b-4281-b951-d872f2087c98