Revert of Remove deprecated extension notification from content_settings (patchset #6 id:240001 of https://codereview.chromium.org/
276573011/)
Reason for revert:
Becuase this CL seems to break linux asan test
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_asan_rel/builds/882
Original issue's description:
> remove deprecated extension notification from content_settings.
>
> Use ExtensionRegistryObserver instead.
>
> R=bauerb@chromium.org
> BUG=354046, 354458
> TEST=unit_tests
>
> Committed: https://crrev.com/
76ad58487578183de69a8caeb48b8a22dccad652
> Cr-Commit-Position: refs/heads/master@{#294768}
TBR=bauerb@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=354046, 354458
Review URL: https://codereview.chromium.org/
553263004
Cr-Commit-Position: refs/heads/master@{#294773}