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When ArcAppListPrefs is destroyed, ExtensionApps doesn't clear the
arc_prefs_. So ExtensionApps still calls arc_prefs_ to remove itself from the
observer list, which could cause the memory issue, when ArcAppListPrefs is
deleted first. Update ArcAppListPrefs to notify the observer,
ExtensionApps, when ArcAppListPrefs is destroyed.
Note: ExtensionApps doesn't depend on ArcAppListPrefs, becuase even if no ARC
on the Chrome OS, ExtensionApps still exists.

BUG=1013850

Change-Id: I389e7b7c2d4e11a5af7f927a1595f75d7b2cc1f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1857840
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705558}