[Extensions] Don't allow overwriting ManifestHandlers

Currently, ManifestHandler::Register() and
ManifestHandlerRegistry::RegisterManifestHandler() allow registering a
new ManifestHandler with a key that has already been associated with
another ManifestHandler, overwriting the current entry.

Remove this behavior, and make ManifestHandlerRegistry DCHECK if a
handler is registered with an already-associated key. This is for a few
reasons:
- We never actually overwrite keys currently
- If we did overwrite a key, it would likely be a bug
- It's conceptually simpler
- It will make removing linked_ptrs from ManifestHandlerRegistry
  simpler.

Bug: 914401

Change-Id: Ie15e9598cc65b8ce3c6b95a666addb41e54d8e0d
Reviewed-on: https://chromium-review.googlesource.com/c/1374115
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616576}
3 files changed