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Introduce an ExtensionsAPIProvider that contains knowledge of
generated APIs and extension features. The ExtensionsClient
contains multiple ExtensionsAPIProviders, and queries these
providers for features and generated API schemas. This allows the
registration of APIs from outside the extensions system, which will
be necessary for moving Chrome Apps API registration outside of
extensions code. This also more effectively wraps the logic relating
to the generated features within a single location, minimizing the
burden for consumers to add/check multiple generated sets.

Create an API Provider for each set of code that generates unique
features and APIs:
- Core Extension APIs (defined in //extensions)
- Chrome Extension APIs (defined in //chrome)
- Shell Extension APIs (defined in //extensions/shell)
- Cast Extension APIs (defined in //chromecast)

When the corresponding ExtensionsClient type is constructed, it
registers the appropriate providers.

Bug: 873937
Change-Id: I198e5e5cbd6ab53bff9e335ea1551bdcfe59f521
Reviewed-on: https://chromium-review.googlesource.com/1172000Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584571}