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This CL introduces no behavior changes.
Now WebAppRegistrar::GetAppIds() uses foreach GetApps().

In follow up CLs:
- rename AllApps() to GetAppsIncludingStubs() (urgent)
- migrate more cases to use GetApps() instead of AllApps() (not urgent)

Implementation details:

AllApps() and GetApps() use protected method FilterApps(filter).
In general, FilterApps() allows to iterate over various subsets from
AllApps().

Internally, Iter::FilterAndSkipApps() filters out unwanted entries.

Bug: 891172
Change-Id: I9a136f9b4f77e77eac28e13d9281453015145925
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1295131Reviewed-by: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818710}