[Extensions Cleanup] Move UnloadedExtensionReason to a new file

Move the UnloadedExtensionReason enum to a new file,
unloaded_extension_reason.h. There's no reason it needs to belong in
extension.h, which is already quite large.

Bug: 1038392
[email protected],[email protected],[email protected],[email protected],[email protected]

Change-Id: I25da6416a322ed8922bb0b55f73cf7b9cd105df8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1982178
Commit-Queue: Devlin <[email protected]>
Reviewed-by: David Bertoni <[email protected]>
Cr-Commit-Position: refs/heads/master@{#729071}
41 files changed