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For incremental conversion of iOS code from BrowserContextKeyedServiceFactories
to BrowserStateKeyedServiceFactories, iOS uses cross-dependencies between both
types of factories.

This CL had two virtual methods to KeyedServiceBaseFactory to convert a context
to the correct type (content::BrowserContext or web::BrowserState) and back so
that we can have BrowserStateKeyedServiceFactories registered into a
BrowserContextDependencyManager (temporary).

Implement the method in all derived classes and refactor the implementation
of GetAssociatedPrefRegistry that no longer needs to be virtual.

BUG=459020,478763

Review URL: https://codereview.chromium.org/1090373003

Cr-Commit-Position: refs/heads/master@{#326782}