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Consider the following circumstance:
- User installs extension Alpha normally (e.g., through the webstore).
  Alpha has Manifest::Location INTERNAL.
- Corp policy pushes out an update that lists Alpha as a required
  extension.

Currently, this behaves fantastically poorly. We validate that the user
is allowed to load an extension on each run of Chrome when loading
installed extensions, and UserMayLoad() will return false if the
extension is required by corporate policy (in order to prevent users
from installing "over" a policy-required extension). This means that
the policy-required extension gets disabled with reason
DISABLE_BLOCKED_BY_POLICY.

Make this slightly less bad by introducing a new policy provider method,
UserMayInstall(). This checks whether the user is allowed to install a
given extension, rather than whether the user is allowed to load it. For
default implementations, UserMayInstall() forwards to UserMayLoad()
(since they should be treated equivalently). However, we can now move
the check for if a Manifest::INTERNAL extension is required policy to
the UserMayInstall() check rather than UserMayLoad(). The effect of this
is to allow the user to load an already-installed extension that's
required by policy, but not policy-installed itself, while still
preventing the user from installing a new copy of that extension.

This is not a perfect solution, since the installed version of the
extension will still be the Manifest::INTERNAL one. This has a number of
implications, including that the extension won't show most policy-
installed indications and won't have access to policy-only APIs. In
extreme cases, the extensions can also be different versions. In the
future, we will need to think about how to handle this scenario more
gracefully.

Bug: 894184

Change-Id: I83c9c305b56f90ea211e4a9b0120b22d601b4fb4
Reviewed-on: https://chromium-review.googlesource.com/c/1327616
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608538}