Revert of Remove deprecated extension notification from ProcessManager. (patchset #2 id:20001 of https://codereview.chromium.org/566863002/)

Reason for revert:
One of the tests crashes with a UAF: http://crbug.com/414245

Original issue's description:
> Remove deprecated extension notification from ProcessManager.
>
> This patch used EntensionRegistryObserver instead of DEPRECATED extension
> NOTIFICATION_EXTENSION_UNLOADED_DEPRECATED removed now.
>
> BUG=411568
>
> Committed: https://crrev.com/1bd52ffda33ffc5614dab8e3f3d677f0a528e763
> Cr-Commit-Position: refs/heads/master@{#294799}

TBR=bauerb@chromium.org,rockot@chromium.org,limasdf@gmail.com,jitendra.ks@samsung.com
NOTREECHECKS=true
NOTRY=true
BUG=411568

Review URL: https://codereview.chromium.org/570073002

Cr-Commit-Position: refs/heads/master@{#294806}
3 files changed