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This CL de-extension-ifies ApkWebAppInstaller, and updates its tests to
be run over both the extension backend and the new web app backend.

BUG=1062145

Change-Id: I6277fc02873eb1b99101cf69eda57db9f332494e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2106035
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750863}