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This CL makes a distinction between the web app installability code path and the
native app (Play) installability code path used by Android using the names
"InstallableWebApp" and "InstallableNativeApp" respectively.

This also rejigs the WeakPtrFactory arrangement to not require the base class
to have subclass only methods.

There are no behavioural changes made by this CL.

Bug: 964314
Change-Id: I1b1d0d3edf91889da4e3caf673208b9a8671fbb4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626056
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663856}