Move more ExtensionService code to ExtensionRegistrar

Adds several functions to ExtensionRegistrar:
* AddExtension
* ReplaceReloadedExtension
* IsExtensionEnabled

EnableExtension and DisableExtension have been augmented to do most of
the work ExtensionService used to do here, and delegate the remaining
work to the new ExtensionRegistrar::Delegate. The delegate can query
policy, update the profile, and other things still part of //chrome.

Design doc: https://goo.gl/trZKep (Google-internal).
First CL:
https://chromium-review.googlesource.com/c/chromium/src/+/671206

Bug: 770879
Change-Id: I7e5f43b458fd722137c0593b826a4b98d115387a
Reviewed-on: https://chromium-review.googlesource.com/677888
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507474}

• chrome/browser/extensions/extension_service.cc[diff]
• chrome/browser/extensions/extension_service.h[diff]
• extensions/browser/extension_registrar.cc[diff]
• extensions/browser/extension_registrar.h[diff]
• extensions/browser/extension_registrar_unittest.cc[diff]