[dPWA] Rename AppRegistrarObserver to WebAppRegistrarObserver

During the BMO migraiton WebAppRegistar had a base class called AppRegistrar, this base class is where the AppRegistrarObserver
got its name.
Now that WebAppRegistrar has taken AppRegistrar's place the
observer interface should be renamed to match.

Bug: 877898
Change-Id: I0902e7c3a232184efc55c532ba9268cfd8916379
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4551403


Reviewed-by: Glen Robertson <glenrob@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1153877}