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AppBannerManager is the only callsite, and it has platform-specific
implementations. IsWebAppInstalledForUrl has platform-specific
implementations as well. Thus it fits nicely into
AppBannerManager{Desktop,Android}.

The rest of installable_utils is not needed by WebLayer, so this
crosses one more file off the componentization TODO list.

Bug: 1147268
Change-Id: I783942d31c993d67bc57ed08b4f2f2541ce911ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537925
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828599}