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Previously, ArcIntentHelperBridge::OnIntentFiltersUpdated() called
OnAppsUpdated() for each observer, but this was probably just confusing.
This CL renames the observer method to fix the inconsistency and to
make it easier to maintain the ArcServiceManager::Observer interface.

This is a follow-up CL for
https://codereview.chromium.org/2487623002/.

BUG=672840
TEST=try

Review-Url: https://codereview.chromium.org/2579083002
Cr-Commit-Position: refs/heads/master@{#439008}