[Extensions] Migrate extensions when per-extension toggle is enabled.

Before this change, when the per-extension toggle features is enabled,
any newly added extensions would automatically start using the
per-extension toggle instead of dev mode to allow user scripts. However,
this didn't handle all the previously installed extensions that need to
be migrated.

After this change any eligible previously installed extensions are
converted to use the per-extension toggle (preference). Eligible means
that the extension is able to use the user scripts API (has been granted
it or can request it). The logic for migrating is:

If the userScripts permission is granted and dev mode is on then user
scripts are allowed, otherwise they are not allowed.

It does this migration in two parts. It handles currently enabled
extensions by hooking into AreUserScriptsAllowed() which is called when
an enabled extension is loaded. It handles all non-enabled extensions by
waiting for the extension system to be ready and then iterating through
all non-enabled extensions to migrate them. In this way we should
migrate all extensions at one time.

The migration is only meant to run once and then never again once it
completes. Any newly installed extension after the migration is complete
should behave the same as before.

Bug: 390138269
Change-Id: I8e377c7b5dd1519a9a09099496bea13adced2e95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6394365
Commit-Queue: Justin Lulejian <jlulejian@chromium.org>
Auto-Submit: Justin Lulejian <jlulejian@chromium.org>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1452610}