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Classes will use ExtensionRegistrar to enable and disable extensions, as
well as add, remove, reload, and terminate them (later CLs).

Currently Chrome's ExtensionService does this, among many other things.
A standalone class for this is preferred because:
* Code outside //chrome uses similar steps to run extensions, so we
  should share this code in //extensions
* ExtensionService already does too much and is very complex

This CL adds EnableExtension() and DisableExtension() to
ExtensionRegistrar. ExtensionService still does some Chrome-specific
work, but later we'll factor that out into a delegate.

Later CLs will add the other extension lifecycle methods, so eventually
ExtensionRegistrar is the only class allowed to change the
ExtensionRegistry (thus the name). We'll also update calls to
ExtensionService methods to use ExtensionRegistrar directly when it
makes sense.


Design doc: https://goo.gl/trZKep (Google-internal).

Bug: 762642
Test: extensions_unittests: ExtensionRegistrarTest
Change-Id: I2910a76f873122fbc405b7cc76c766b00a8b216f
Reviewed-on: https://chromium-review.googlesource.com/671206
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505206}